The influence of intrinsic formability parameters on the sheared-edge ductility of 120K-class AHSS

With enduring versatility, advanced high-strength steels (AHSS) play an increasingly important role in securing automotive safety, performance and sustainability targets in future automotive architectures. However, local formability (resistance to cracking) is of specific relevance to AHSS, and various intrinsic fracture-related concepts have emerged with the goal of understanding and improving the local formability of AHSS (sheared-edge ductility, bendability). This analysis is focused on the relationships between intrinsic formability parameters and other laboratory-scale performance indicators—i.e. the ISO 16630 hole expansion ratio (HER). Included is the AHSS material with minimum tensile strength designation above 1.2GPa: 120K-class 3rd generation AHSS. Additionally, the impact of material property uniformity on HER, and local toughness concept based on the true fracture strain is explored.


Introduction
Concerns related to lightweighting in the automotive industry are increasing due to the need for improving automotive fuel efficiency, meeting stringent CO2 emission standards, and complying with safety regulations.To address these demands, advanced high strength steel (AHSS) is widely applied in automotive components due to its favorable strength-to-density ratio.Despite its advantages AHSS faces challenges related to local formability failure modes, such as edge cracking, which are difficult to predict using conventional forming limit curves due to the absence of observable thinning or necking before fracture.[1] To evaluate sheared-edge ductility, it is common practice to perform hole expansion testing per ISO 16630.However, the hole expansion ratio (HER) is regarded as a relative "local formability" parameter due to the intra-laboratory scatter controversy, and the ratio itself has a limited meaning as a practical failure criterion.Thus, to evaluate the performance of AHSS grades, a practical performance classification system considering both global and local formability based on true fracture strain measurement was proposed and explored continuously by steelmakers, automakers and international industry consortiums.[2] The paper aims to evaluate the performance of AHSS materials using the intrinsic global and local formability parameters derived from standard uniaxial tension tests.[3] Additionally, a toughness concept based on the true fracture strain value was investigated to explore the impact of material property uniformity on HER.  3 shows the testing parameter for the hole expansion test in accordance to the ISO 16630 standard.All testing was carried out in the same lab with a universal sheet metal testing machine.First, the specimen was pierced by using the correct tool for even clearance for different thickness.Then, the specimen was expanded with a conical punch until through-the-thickness crack was observed.Note that 0.9mm thickness is not in the range of the thickness the standard states.Hance were used to derive the true fracture strain value and the intrinsic formability parameters for this study.[3] Figure 1 shows the locations of the thickness and the width measurement.For the thickness, two points at the edges, one point at the center and two points at the quarter width positions were measured.For the fracture width, a single measurement at mid-thickness was done with an optical microscope.Finally, fracture area (Af) was determined by the methods defined in Table 4.All measurements for the fracture surface were made with respect to the projected plane normal to the tensile axis.

Results and discussion
3.1.Validation of measurement method.In order to define which fracture area measurement method to use, two types of tensile test specimen, ASTM E8 subsize and EN ISO 6892-1 were prepared from a 120K grade steel sheet regarding the sheet rolling direction.The considered orientation of the specimen are 0°,45° and 90° respectively.After tensile testing, the fracture area was measured for each according to the mentioned method.Then, the true fracture strain(TFS) was derived with the acquired measurement for comparison with the method described by Hance. [3] Where A0 is the tension test specimen cross-section area before testing and Af is the projected fracture surface area after fracture.5 is the comparison result for the effect of test specimen type and fracture measurement method on true fracture strain values for 120K grade steel.Overall, Method B showed similar variation as Method C, but note that Method C is conceptually more accurate and conservative method.

Impact of material property uniformity.
One of the crucial assessments at the development stage is to evaluate the material property uniformity for quality control and problem identification.ASTM E8 subsize samples in the transvers orientation were acquired from various width locations for assessment, as shown in Figure 2. As presented in Table 6, material property variability across the coil-width blank was observed for the 120K prototype grade steel, particularly evident for yield and tensile strength.Thus, the impact of cross-width property uniformity on the intrinsic formability parameters and the correlation between the local formability performance indicator, HER was investigated.The intrinsic formability parameters described by Hance were used, as in Table 7 [3].Note that local/global strain ratio and formability index are strain-based parameters, and the performance index is based on both strain and the ultimate tensile strength.The calculated intrinsic parameter results, along with the measured hole expansion ratio is summarized in Table 8.From the above results, the HER results had high linear correlation with the global property, yield strength as shown in Figure 4.For a better interpretation of the formability performance, simplified toughness approach regarding the true fracture strain was considered, as significant correlation between HER and fracture toughness was verified by previous studies.[5,6] Table 9 is the summary of the formability assessment results with the toughness results included.From the result, when yield strength and TFS are considered together, the correlation with HER is substantially higher than for TFS alone.Note that these data represent various positions across a single coil-width blank.In addition, relatively significant linear relationship was found between the local toughness concept and the hole expansion ratio than for yield strength or TFS alone when different range of grades were considered together(not detailed due to confidentiality reasons).

Conclusion
In conclusion, this study examined the intrinsic formability parameters and their influence on the sheared-edge ductility of 120K grade steel regarding the material property uniformity.The analysis focus on the investigation of the relationship between the formability parameter and laboratory-scale performance indicators such as the ISO 16630 hole expansion ratio.Additionally, toughness concept was considered for further investigation.The following conclusion was drawn from this analysis: • The observed cross width variability observed in global material property influenced both intrinsic parameters and the laboratory-scale performance indicators.
• Through the formability assessments, local toughness and the hole expansion ratio portrayed significant linear relationship than for true fracture strain alone.
• The above investigation and method will be expanded for different AHSS grades for future work.

Figure 1 .
Figure 1.Schematic representation of a tension test specimen fracture surface.Five thickness and one width measurement locations are as shown[3]

Figure 2 .
Figure 2. Schematic representation of a coil-width blank for assessment of material property uniformity.Samples were acquired from each highlighted zone, as indicated.

Figure 3 .
Figure 3. Test result comparison cross the width.

Figure 6 .
Figure 6.Various measure of AHSS performance

Table 1 .
MaterialThis analysis includes a prototype AHSS material with minimum tensile strength designation above 1.2GPa.The basic material description is given in Table1.Basic material description

Table 2 .
Nominal tensile test specimen dimensions

Table 3 .
Hole expansion test specimen specification 2.3.Fracture area measurement method.Various fracture area measurement methods described by

Table 7 .
Intrinsic parameters based on true fracture strain value