PSI - Issue 68

Eren Çelikses et al. / Procedia Structural Integrity 68 (2025) 1045–1050 E. C¸ elikses et al. / Procedia Structural Integrity 00 (2024) 000–000

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microstructure of DP steels. Alternatively, DP steels can also be produced directly from the rolling heat in hot strip mills (Ray, 1984), providing a versatile production pathway. As demonstrated in Marder (1982), the mechanical properties of DP steels depend on various parameters, such as chemical composition, phase volume fractions, and grain size etc. Numerous studies have been conducted to optimize the mechanical properties of DP steels. In Rocha et al. (2005); Kim et al. (2009); Peranio et al. (2010); Bandi et al. (2024) the e ff ects of heating rate, soaking temperature, soaking time, and the final slow cooling temperature on the material’s microstructure and mechanical properties are investigated. Another critical factor influencing the mechani cal behavior of DP steels is the distribution of martensite islands, which may align along the rolling direction to create a ”banded” structure. Although the mechanisms behind this alignment are not fully understood, several hypotheses have been proposed. As discussed in Thompson and Howell (1992), elements such as Mn and Si are rejected from δ -ferrite dendrites with decreased cooling rate, concentrating in interdendritic areas. This solute distribution persists during the δ -to- γ transformation, establishing the foundation for microstructural banding. Recent studies (Krebs et al., 2011) have examined how ferrite transformation modes impact banding. In segregated structures, allotriomorphic ferrite grows in Mn-poor areas, enhancing banding, while acicular ferrite shows isotropic growth. The e ff ects of banded structures on the mechanical properties of dual-phase (DP) steels have been extensively studied. The negative impact of banding on DP steel properties is highlighted in Shirasawa and Thomson (1987), noting that non-banded structures exhibit superior ductility and tensile strength. The influence of banding on shear band formation is examined in Dastur et al. (2024), observing that equiaxed microstructures contain a greater number of shear bands, whereas shear bands tend to be more localized in banded structures. Furthermore, the impact of banding on the failure initiation mechanism is investigated in Ramazani et al. (2014). It is discussed that equiaxed structures experience failure initiation at higher plastic strains compared to banded structures. Various methods have been developed to mitigate the adverse e ff ects of banded structures on mechanical properties, including optimizing alloying elements (Slater et al., 2022) and selecting appropriate heat treatment procedures (Lan et al., 2021). The investigation of the influence of microstructural parameters on the macroscopic forming performance requires detailed modeling using representative volume elements and proper boundary conditions (see e.g. Yalc¸inkaya et al. (2021); Acar et al. (2022)). In this study, two similar grade (DP800) steels with banded and dispersed martensite structures are examined through mechanical testing and numerical simulations. Steels are manufactured through di ff erent procedures to achieve desired microstructures. They are then characterized with Nakazima and hole expansion experiments. Fur ther numerical analysis is performed using the microstructures generated from micrographs of the specimen to gain a deeper understanding of how banding a ff ects stress distribution and deformation mechanisms within the material. DP800 steels, produced under two di ff erent sets of manufacturing parameters, are subjected to microscopy exami nation, mechanical testing to characterize the materials. Optical microscopy examination is performed on the DP800 samples, which are etched using nital (%2) solution to reveal the microstructural details. The etching process high lights the distinct phases within the steel, enabling a clearer observation of the ferrite and martensite distribution. Microscopic images are presented in Fig. 1, showing that the first sample exhibits martensite in a banded morphology, while the martensite in the second (dispersed) DP800 sample forms isolated grains within the ferrite matrix. The uniaxial tensile test is conducted in accordance with ISO 6892-1 to assess key mechanical properties of the steels, including yield strength, ultimate tensile strength, and elongation at fracture. The tensile test is conducted with the rolling direction aligned at 0°. A strain rate of 0.00025 s − 1 was applied to ensure precise determination of the yield strength. Yield strength is measured using the 0.2% o ff set method, and the ultimate tensile strength is recorded as the maximum stress prior to the onset of necking. The results are listed in Table 1. Despite their microstructural di ff erences of the two samples, their mechanical properties in tensile tests are found to be similar. In the hole expansion test, three samples measuring 100 mm x 100 mm are cut with guillotine shears from two materials of di ff erent thicknesses. A 10 mm diameter hole is punched into the center of each sample using a 60-ton capacity eccentric press, with the cutting clearance adjusted accordingly. The tests were conducted using a 60°conical punch at a speed of 0.75 mm / s without lubrication. During each test, the force acting on the punch and the punch stroke data are recorded. According to ISO 16630, the tests are stopped once a crack had fully propagated through the 2. Experiments