Materials Science Forum Vol. 1174

Abstract: To produce functional cups by press forming, clad cups with a corrugated structure with voids like the cross section of corrugated cardboard were formed. Deep drawing, which is one type of press forming, is a plastic processing technology that forms thin sheets into three-dimensional containers. In the experiment, pure titanium TP270 and ultra-low carbon steel SPCC were used as test materials. The blank sheet thickness was 0.3 mm and the diameter was 80 mm to 90 mm. To form the corrugated cup, the roller ball die with steel balls installed on the shoulder of the die was prototyped. The steel balls were made of bearing steel JIS-SUJ2 and had diameters of 6.4 mm and 7.5 mm. The corrugated clad cup was formed by the composite die combined with a conventional die. Three conventional dies and two roller ball dies were used to obtain two corrugated layers with voids. The lubricant was a tool oil containing molybdenum disulfide powder. The sheet thickness strain distribution and residual stress distribution of the cup were evaluated. No destruction of the cup occurred during deep drawing. A regular wavy structure was observed in the cross section of the cup. The maximum reduction in the cup thickness was approximately 10 %. The residual stress on the outside of the cup was tensile stress from the bottom to the opening of the cup. The composite die made it possible to form a functional cup.

Abstract: To increase the safety of steels in high performance cases like crash energy absorption, even better properties of the materials are necessary. To advance this research, a TWIP and a TRIP steel were combined in a laminated composite via roll bonding at 450 °C with the goal of using accumulative roll bonding (ARB) in later research to further enhance the properties reaching an ultra-fine-grained material. Two different TWIP layer thicknesses (2 mm and 3 mm) were experimentally roll bonded with a 3 mm thick TRIP layer each using a 4-high rolling mill. A modular Python-based simulation incorporating coupled solving of ordinary differential equations of the temperatures and the horizontal stress changes of the layers were implemented to predict deformation and bonding behavior. Simulated results matched well with experimental data in terms of final geometry and temperature, while roll force deviations indicated the need for the refining of the used model. Furthermore, experimentally asymmetric layer relationships at the beginning and the addition of a thin (10 µm) Ni interlayer were found to enhance bond strength in high-strength steel laminates.

Abstract: The stretch flangeability of ferritic steel grade with tensile strength ≥1 GPa in hole expansion tests can be significantly improved by using the wire electrical discharge machining (W-EDM) process for hole-making instead of conventional punching tools. This improvement is attributed to the notably enhanced cut edge quality of the machined holes. In this study, the average hole expansion ratio (HER) of a novel 0.1C-0.3V-0.25Mo-0.08Ti-0.08Nb steel increased from 24% to 91% when W-EDM was used in hole preparation. A comparison between the fractured surfaces of punched and W-EDM-machined holes after HER testing revealed different failure mechanisms in the steel sheet. At the onset of cracking, fractures in the W-EDM specimens exhibited ductile behavior, whereas quasi-cleavage fracture was observed in the punched specimens. Based on texture measurements and metallographic investigations, it was concluded that reducing the intensity of the adverse shear texture component {112}<111> near the steel sheet surface and eliminating microstructural constituent variations improved the stretch flangeability of the Ti-Mo-V-Nb steel in both hole-making processes.

Abstract: In-situ high-energy synchrotron X-ray diffraction was employed to track phase evolution in a medium-carbon (0.4 wt.% C) advanced high-strength steel processed via quench-and-partitioning (Q&P). Real-time diffraction data, captured during quench stop temperatures of 200 °C (QT200) and 240 °C (QT240), followed by a 1000 s holding time at the partitioning temperature of 300 °C, revealed precise phase fractions during the Q&P. However, the retained austenite in both process routes produced comparable retained-austenite fractions at room temperature —23% for QT200 and 21% for QT240—the higher quench-stop temperature generated three times more fresh martensite (15% vs. 5%). The mechanical properties were examined by tensile tests, showing that the lower fresh-martensite content in QT200 promotes progressive, strain-induced austenite transformation, delays necking, and yields a uniform elongation of 6.9%. By contrast, QT240 reaches a higher ultimate tensile strength (around 2023 MPa vs. 1984 MPa) and yield strength (about 983 MPa vs. 938 MPa) at the expense of ductility (around 4.7% uniform elongation). In both conditions, the TRIP effect is active, but its contribution is curtailed by the presence of fresh martensite. The present study thus establishes a quantitative link between in-situ phase evolution pathways and the resulting strength–ductility balance, providing guidance for tailoring Q&P processing route in medium-carbon advanced high-strength steel for crash-critical automotive applications.

Abstract: Perforated metal sheets with numerous holes created by punching are used for various purposes, ranging from automotive parts to daily kitchen products, in applications such as ventilation, soundproofing, shielding from sound and light, and weight reduction. However, the plastic forming of thin metal sheets has generally been limited to materials of uniform thickness and quality. It has not been the subject of significant technological development or research. Most current products have flat shapes or are simply bent, and there are few precedents for machining methods that apply strong forces to the hole edges. In the drawing process, in which a strong force is applied to the hole edge, changes in tensile and compressive deformation occur due to discontinuities in deformation resistance. This results in non-uniform sheet thickness caused by a combination of deformation in the holes and no deformation in the surrounding sheet. Therefore, clarifying the drawing characteristics of perforated sheets will contribute to progress in plastic forming.

Abstract: Advanced high-strength steels (AHSS) can offer an excellent combination of high strength and light weight for applications including cold forming. These steels may be used in press braking and profile and section manufacturing. The aim of this work is to study the mechanical properties and microstructure of steel used in the manufacturing of roll-formed profiles with decreased radii. In this study, ferritic feedstock steel with 450 N/mm² yield strength, coiled at room temperature, and the sections with wall thicknesses of 7.1 mm and 12.5 mm are studied with middle line radii of 7.5 mm and 13 mm, respectively. Mechanical properties and impact toughness, especially considering the corners of cold-formed profiles, without and with aging (250 °C for 1 h), are studied. Cold deformation on corners is influenced by thickness and tensile strength in aged conditions. Impact energies are at a high level at-40 °C and-60 °C, and aging does influence the toughness but not on the hardness.

Abstract: In this study, the electromagnetic heating of a steel sample in dilatometer was modeled with finite element method. The model was developed to simulate electromagnetic heating process of Linseis DIL L78 DQT/RITA Quenching & Deformation dilatometer, using the dimensions, current and frequency measured from the dilatometer for model validation. Thermophysical and electromagnetic behaviour of a steel is highly temperature-dependent, necessitating the temperature dependent material properties of the test material. The goal of this study was to replicate the behaviour of the electromagnetic heating in the dilatometer as accurately as possible. In electromagnetic heating the material properties have a significant impact on the efficiency of the heating process. The material must be electrically conductive to allow generating the electric current caused of a changing magnetic field which forms the electric field on the surface of the heated material. Material properties, which vary with temperature, were defined in the model as a function of temperature to ensure realistic thermophysical behaviour of the simulated part. Two different analysis solvers were used for electromagnetic and heat transfer analysis. The model was validated using measured data from the dilatometer.

Abstract: The effects of silicon (Si) addition and continuous annealing (CA) parameters on the microstructure and mechanical properties of low carbon Nb-Ti steels were investigated. Steels with and without Si were subjected to CA simulations, varying annealing temperature, line speed (LS), and cold work (CW) levels. Low-temperature thermomechanical controlled processing (TMCP) during hot rolling produced a fine polygonal ferrite matrix with uniformly distributed, spherical cementite - finer and more homogeneously dispersed in the Si-containing steel. Surface oxides in the as-rolled Si steel consisted mainly of wüstite and magnetite, with no deleterious hematite or fayalite observed due to high temperature descaling. Recrystallization during CA began near 650°C and completed above 780°C but was delayed by Si addition, higher line speeds, and moderate cold work. The final ferrite grain size remained fine, averaging 4–5 μm, across a broad annealing temperature range, aided by effective grain boundary pinning from carbonitrides. In the 690–760°C annealing range, the Si-containing steel exhibited increased strength due to solid solution strengthening, carbonitride precipitation hardening and restricted recrystallisation. Despite this, elongation was preserved through the formation of fine, soft, ductile, uniformly dispersed spherical cementite (Fe₃C) in the Si steel. Higher levels of cold work reduced strength slightly after annealing above 780°C but improved elongation due to full recrystallisation and coarsening of NbTi (C,N) particles.

Abstract: The Harmonic Structure [1] is a novel design concept that facilitates the engineering of metallic materials to achieve enhanced mechanical performance. The Harmonic Structure is composed of soft, coarse-grained regions, designated as the Core, which are surrounded in three dimensions by an interconnected network of hard, ultra-fine grain regions, referred to as the Shell. The interaction in these core/shell regions produces a synergistic effect during plastic deformation, resulting in superior mechanical properties that are of great significance. The distinctive network configuration of the Harmonic Structure enhances the dislocation density within the coarse-grain regions in contact with the interface through stress partitioning, thereby accelerating the work hardening rate and consequently enhancing the strength. This phenomenon is referred to as Hetero Deformation Induced (HDI) strengthening [2]. The fabrication of HS material is achieved through the application of mechanical milling (MM) to the powder, which results in the formation of a deformed layer on the surface of the powder and the creation of bimodal structured particles. However, a notable constraint of the MM process is its extended time requirement to attain the desired bimodal structure. In contrast, the bi-modal milling (BiM) technique involves the controlled mechanical milling of coarse and fine powders in conjunction with each other, with the objective of forming a layer of fine powders of a specified thickness over the coarse particles. The most advantageous aspect of bi-modal milling (BiM) is not only its reduced processing time, but also its superior ability to control the thickness of the surface deformation layer.