Search Results (80)

Resistance spot welding (RSW) is the primary joining technology for automotive advanced high-strength steels (AHSSs), where the inherent severe thermal cycles profoundly alter the heat-affected zone (HAZ) microstructure, leading to localized variations in mechanical properties. Characterizing the spatial gradients in microstructure and the resulting mechanical properties remains a major challenge in weld failure analysis due to the small size of the HAZ and its complex thermal history. In this study, the HAZ of two prominent AHSS grades, a first-generation press hardening steel (PHS1500) and a third-generation quenching and partitioning steel (Q&P1180), was physically simulated using a Gleeble^® 3180 thermomechanical simulator to achieve precise control over the localized thermal cycles. The investigation first evaluated the role of thermal cycle duration, governed by the welding time parameter (300 ms vs. 800 ms), on the microstructural evolution of the PHS1500 steel. Increasing the weld time from 300 ms to 800 ms reduced the cooling rate under the nominal 1400 °C condition from approximately 3000 K/s to 2500 K/s; however, no marked change was observed in the overall microstructural and hardness trends within the investigated range. Subsequently, using the 300 ms thermal profile as a reference baseline, a comparative metallurgical study was conducted between PHS1500 and Q&P1180. Under the same 300 ms thermal history, the maximum hardness reduction relative to the corresponding base material was approximately 42% for PHS1500 and 12% for Q&P1180. The hardness minima were located within FE-estimated temperature ranges close to the Ac₁ region for PHS1500 and around 600 °C for Q&P1180, respectively. This comparison highlighted the distinct microstructural responses of the two generations across the upper-critical (UCHAZ), inter-critical (ICHAZ), and sub-critical (SCHAZ) zones. Moreover, microhardness profiles were correlated with the microstructural findings, establishing a correlation among the simulated thermal history, the observed microstructural evolution, and localized mechanical performance. Full article

Direct hot press forming (DHPF) of Zn-coated press-hardening steel (PHS) has not been widely adopted by industry due to liquid metal embrittlement (LME), which occurs when coated steel is hot stamped above the Fe-Zn peritectic temperature (~782 °C). In the present study, low-temperature hot stamping was performed on a 2.0 wt% Mn PHS to avoid LME by preventing liquid zinc formation during plastic deformation while achieving target mechanical properties (yield strength (YS) ≥ 1100 MPa and ultimate tensile strength (UTS) ≥ 1500 MPa) and preserving corrosion performance. The enhanced hardenability, indicated by a critical cooling rate (CCR) of 10 °C/s, enabled a predominantly martensitic microstructure following DHPF at 550–700 °C. Tensile testing of samples extracted from U-shaped panels yielded similar results for uncoated and Zn-coated samples, with a YS of ~1170 MPa, a UTS of ~1600 MPa, a uniform elongation (UE) of 0.05, and a total elongation (TE) of 0.09, demonstrating the preservation of baseline mechanical properties in the coated samples. Microstructural analysis confirmed the absence of LME-induced substrate cracking. Additionally, XRD, SEM-BSE, and EDS analyses confirmed Γ-Fe₃Zn₁₀ formation in DHPF galvanized coatings, with volume fractions averaging ~0.6, well above the critical value of 0.15, irrespective of the DHPF temperature, demonstrating the formation of a cathodically protective coating microstructure. Full article

Large-scale asymmetric doubly curved thick shells made of high-strength steel are key components in deep-sea and nuclear pressure vessels. Integral pressing is an important manufacturing method for such components, but it still faces two major challenges: excessive clamping force and poor springback predictability. To address these issues, this study conducts a combined experimental and numerical investigation on a 16 mm thick Q890 high-strength steel plate. First, the through-thickness plastic gradient and cyclic stress–strain response of the material were characterized, and a mixed hardening model incorporating through-thickness gradient plasticity was established. To suppress the lateral force induced by the asymmetric geometry, a blank positioning strategy was proposed, reducing the lateral force to below 2 t (Reduced by 65%). More critically, a striking phenomenon is revealed: during the final 1 mm of the clamping stroke, the forming force surges abruptly from approximately 680 t to 5090 t. Detailed analysis identifies the root cause as the synergistic effects of a sharp increase in contact area, a drastic rise in frictional resistance, and the onset of localized upsetting in regions already in contact with the die. To suppress the load surge while maintaining forming accuracy, a normal-direction over-compensation strategy was proposed. By deliberately increasing the normal compensation, the blank retains a bending-dominant deformation mode at the target clamping position, thereby avoiding the critical contact expansion, frictional buildup, and localized upsetting that trigger the force surge. Through iterative simulations, the optimal die surface is determined, achieving a forming force below 1000 t with a simulated shape deviation within 0.96 mm. Experimental validation using a purpose-built die on a 1000 t press successfully produces the shell with a maximum profile deviation of 1.67 mm, meeting high-accuracy requirements. This work establishes a new paradigm for low-load, high-accuracy forming of thick high-strength steel shells by actively managing contact evolution and deformation mode via normal-direction over-compensation, offering a practical pathway to one-shot tryout success for critical pressure hulls. Full article

The article presents the results of experimental studies on the possibility of predicting the efficiency of CO₂ mineralization using metallurgical wastes (MWs) from the perspective of chemical thermodynamics and on identifying, accordingly, promising MWs for the production of construction materials and products. The study examined MWs from major Russian iron and steel producers, namely: blast furnace, electric steelmaking, ferroalloy, converter steelmaking slag, as well as nepheline slag, a by-product of nepheline ore processing for alumina. The CO₂ binding capacity of MWs was determined using experimental samples fabricated by semi-dry pressing of MW powders, followed by curing them in a gas atmosphere with an CO₂ concentration of 80% vol. It was found that the investigated MWs are capable of absorbing and binding CO₂, thereby improving their physical and mechanical properties. Experimental samples made from nepheline slag bind 11.3 to 12.0 wt.% of CO₂; samples from steelmaking slags: up to 9 wt.% or more; and samples from blast furnace dump slag: approximately 5.5 wt.% At the same time, the compressive strength of samples from steelmaking slags exceeds 100 MPa, that of samples from nepheline slag approaches 80 MPa, and that of samples from blast furnace dump slag exceeds 50 MPa. It has been established that predicting the efficiency of CO₂ mineralization by metallurgical wastes based solely on chemical thermodynamics is not entirely accurate. To develop a preliminary forecasting model for the carbonate hardening potential of various MWs, further studies are needed to identify additional key factors influencing the carbonate hardening process of MWs. Full article

Hydrogen embrittlement (HE) critically limits the application of ultra-high-strength press-hardened steels (PHS) in hydrogen-containing environments. This study investigated the effect of Nb microalloying on HE resistance of 1500 MPa-grade PHS. Even with higher hydrogen contents, steel 1500Nb exhibits better HE resistance than steel 1500. The results show that Nb addition plays effective role in grain refinement, mitigating stress concentration, and effectively postponing the initiation of intergranular cracks under hydrogen-charged conditions. Additional, hydrogen diffusivity in 1500Nb steel is lower than 1500 steel, attributed to both grain refinement effect and solute drag effect of Nb. Full article

In alignment with the European Union’s 2050 carbon-neutrality targets, the automotive industry is intensifying efforts to adopt lightweight materials that ensure structural integrity without compromising safety. Press-hardened steels (PHS), offering a combination of ultra-high strength and formability, are at the forefront of these developments. Standard PHS grades rely on Ti–B microalloying; however, further alloying with Nb and V has been proposed to enhance hydrogen embrittlement resistance via microstructural refinement and hydrogen trapping. This study investigates hydrogen transport and mechanical degradation in a Ti–Nb–V microalloyed PHS compared to a conventional Ti-only 22MnB5 grade. Electrochemical permeation, thermal desorption, and mechanical testing were employed to characterize hydrogen diffusivity, solubility, and trapping mechanisms. The Ti–Nb–V variant demonstrated lower hydrogen diffusivity, higher solubility, and improved resistance to delayed fracture, attributable to the presence of fine NbTiV precipitates. Full article

In order to determine the causes of crack defects in Mn18Cr2 high-manganese wear-resistant steel plates, this paper conducted a systematic analysis of the steel plates’ microstructure, chemical composition, and hardness via metallographic microscopy, field-emission scanning electron microscopy, and Vickers hardness tester. The results indicated that there were folded cracks on the surface of the steel plate. The interior of the cracks was oxidized, and inclusions were observed in the crack gaps. A significant difference in the contents of Mn and Cr elements was detected at the defect locations, indicating that very obvious long-range diffusion of Mn and Cr elements had occurred during long-term high-temperature oxidation. The crack defects on the surface of the steel plate were related to the inheritance of the original cracks on the surface of the cast billet before rolling. There were cracks on the surface of the cast billet; the oxide scale and inclusions inside the cracks had not been completely removed. Multiple passes of rolling led to the cracks and oxide scale being pressed into the steel surface, thereby forming folding defects. The fine grain strengthening and deformation twinning generated by rolling deformation formed the hardened layer on the surface, resulting in higher surface hardness than core hardness. The austenite grain size inside the steel plate was in the range of 23–30 μm, and the hardness was around 275 HV. The grain size near the surface of the steel plate was around 10 μm. The surface hardness was 351 HV, which was higher than the core hardness of the steel plate. Full article

This study reports the successful fabrication and application of floatable syntactic foams derived from fine magnesium powder (<45 µm) utilizing expanded perlite (0.25 g/cm³, 0.2–0.4 mm) as the pore former. Sample disks with densities as low as 0.9 g/cm³ were produced via the classical press and sinter process. To ensure reasonable mechanical properties, the specimens were formed under a pressure of 200 MPa in a hardened steel die, followed by high-vacuum sintering (~3 × 10⁻⁶ torr) at 640 °C for 1 h. The resulting foams exhibited sufficient mechanical strength to allow for precision machining into a microboat. We demonstrated their potential use as a Marangoni-induced microswimmer. Spontaneous locomotion was observed when ethanol was used as a propellant, which generates a surface tension gradient between the upper and rear parts of the swimmer. The microboats achieved propulsion speeds of approximately 160 mm/s when propelled by a 95% ethanol + 5% ink mixture. Using a small volume (~4 µL) of the alcohol mixture, the swimmer could cover distances exceeding 350 mm. Full article

In this study, sheets of experimental high-carbon low-density steels (LDSs) with a thickness of 1.7 mm were processed in a combined tool designed for press-hardening. Press hardening, also known as hot stamping or hot press forming, is a manufacturing process used to create car body parts with exceptional mechanical properties and safety standards. These components often require tailored properties, meaning different mechanical characteristics in various parts of the component. LDSs have a lower specific density than conventional steels, so their use would be particularly suitable in automotive applications. Combined tools achieve distinct mechanical properties within a single part through thermomechanical processing. Simultaneous forming and heat treatment create tailored zones of high strength and ductility within the sheet metal. The hardened zone provides crashworthiness, while the more ductile zone absorbs kinetic energy and converts it into deformation energy. Hot stamping enables forming complex geometries from high-strength sheets with limited cold formability, a capability that can also be exploited for the aluminium-alloyed LDS under investigation in this work. Three different high-carbon LDSs with differences in chemical composition were subjected to this experiment, and the hardness, microstructure, and mechanical properties of the two areas of each sheet were evaluated. The aim is to determine their suitability for processing by press hardening and to try to achieve tailored properties (i.e., differences in ductility and strength across one part) as in a typical representative of 22MnB5 boron steel, where a strength limit of 1500 MPa at 5% ductility is achieved in the cooled part and 600 MPa at 15% in the heated part. Tailored properties were also achieved in the investigated LDS, but with only relatively small differences between the two tool areas. The omega profiles were produced by press hardening without visible defects, and it was possible to process the steels without any difficulties. Full article

High manganese steel can improve its microstructure after aging treatment, which is beneficial for enhancing strength, toughness, and wear resistance. This study aims to explore the effect of aging treatment on mechanical properties and wear resistance of high manganese steel (containing 25% Mn, called Mn25 steel) by designing different aging temperatures (450 °C, 500 °C, and 550 °C) with the same aging time (1 h). The results indicated that with the increase in aging treatment temperature, the surface hardness of Mn25 steel first increased and then decreased, but was still higher than that of untreated Mn25 steel. In addition, the impact toughness of steel decreased first and then increased with the increase in aging temperature, with the optimal hardness and impact toughness exhibited at 550 °C. The impact abrasive wear test results showed that the weight loss of Mn25 steel decreased with the increase in aging treatment temperature. After aging treatment at 550 °C, the weight loss is the lowest, which shows the optimal wear resistance performance. Under a high-impact load of 5.0 J, the hardness increased by nearly 49.96% after impact abrasive wear, and the effective hardening layer of the steel was the thickest, about 3800 μm. This is mainly related to the best match between the hardness and impact toughness of high manganese steel after aging treatment. The wear morphology is often caused by various wear mechanisms working together to cause the wear loss of Mn25 steel during the impact wear process. The wear morphologies of the Mn25 steel were mainly characterized by press-in particles, furrow, spalling, and strain fatigue. Through experimental analysis, a suitable aging treatment process has been determined, providing a theoretical basis for the practical application of high manganese steel. Full article

An ultra-high-strength press-hardening steel (PHS) and a high-strength dual-phase steel (DP) were butt-joined by the gas metal arc welding (GMAW) process, aiming to assess the effects of a high heat input welding process on the structure-property relationship and residual stress. The post-weld microstructure, the microhardness profile, the tensile behavior, and the experimentally obtained residual stresses (by x-ray diffraction) of the steels in dissimilar (PHS-DP) and similar (PHS-PHS, DP-DP) pair combinations have been analyzed. Results indicated that the ultimate tensile strength (UTS) of the dissimilar pair PHS-DP achieves a similar strength to the DP-DP joint, whereas the elongation was similar to that of the PHS-PHS weldment. The failure location of the tensile specimens was expected and systematically observed at the tempered and softer sub-critical heat-affected zone (SC-HAZ) in all welded conditions. Compressive residual stresses were consistently observed along the weldments in all specimens; the more accentuated negative RS were measured in the PHS joint attributed to the higher volume fraction of martensite; furthermore, the negative RS measured in the fusion zone (FZ) could be well correlated to weld restraint due to the sheet anchoring during the welding procedure, despite the presence of predominant ferrite and pearlite microstructures. Full article

In the automotive industry, structural components are often produced via press hardening, enabling rapid production and the use of ultra-high-strength steels. In this process, steels are heated to an austenitic state and are then formed and quenched in rapid succession. The initial steel that enters the press-hardening production line varies, where the microstructure is a result of previous production steps. This work was performed to investigate the possible effects of the initial microstructure on the final mechanical properties for rapidly quenched samples. Although the initial microstructure is transformed during austenitization, the steel can still be affected by its prior history. Steels with three different initial microstructures were evaluated, with only minor variations in chemical composition and thicknesses. The Lankford coefficients and the failure strains were dependent on the orientation of the samples. However, for a given orientation, there were only minor variations between the different steels with respect to anisotropy, strength, and ductility. The anisotropy could be correlated with the microstructure through the calculation of Taylor factors based on measurements using electron backscatter diffraction. The minor influence from the initial steel microstructure on the final mechanical properties indicates robustness suitable for mass production. Full article

The automotive industry is currently in a paradigm shift transferring the fleet over from internal combustion vehicles to battery electric vehicles (BEV). This introduces new challenges when designing the body-in-white (BIW) due to the sensitive and energy-dense battery that needs to be protected in a crash scenario. Press-hardening steels (PHS) have emerged as an excellent choice when designing crash safety parts due to their ability to be manufactured to complex parts with ultra-high strength. It is, however, crucial to evaluate the crash performance of the selected materials before producing parts. Component testing is cumbersome and expensive, often geometry dependent, and it is difficult to separate the bulk material behaviour from other influences such as spot welds. Fracture toughness measured using the essential work of fracture method is a material property which has shown to be able to rationalise crash resistance of advanced high-strength steel (AHSS) grades and is thereby an interesting parameter in classifying steel grades for automotive applications. However, most of the published studies have been performed at quasi-static loading rates, which are vastly different from the strain rates involved in a crash. These higher strain rates may also lead to adiabatic self-heating which might influence the fracture toughness of the material. In this work, two PHS grades, high strength and very high strength, intended for automotive applications were investigated at lower and higher strain rates to determine the rate-dependence on the conventional tensile properties as well as the fracture toughness. Both PHS grades showed a small increase in conventional mechanical properties with increasing strain rate, while only the high-strength PHS grade showed a significant increase in fracture toughness with increasing loading rate. The adiabatic heating in the fracture process zone was estimated with a high-speed thermal camera showing a significant temperature increase up to 300 °C. Full article

Solutions and new processes are continually being developed to produce components demonstrating high strength and elongation. This paper focuses on medium manganese steel with a composition of 0.2% carbon, 3% manganese, and 2.15% aluminium (by weight percent). The mechanical properties of the steel and the effect of aluminium and manganese on the microstructure are investigated. The steel sheets are shaped into omega profiles using a press tool, followed by the intercritical annealing of the samples to enhance their ductility. Before the experiment, the anticipated values were a tensile strength (UTS) of approximately 1100 MPa and elongation within 30–35%. A key objective was to achieve a microstructure that incorporates residual austenite. The experimental parameters were carefully derived from an extensive exploration to identify potential weaknesses in the experiment. The main parameters selected were the intercritical annealing (IA) temperature and IA dwell time. The results revealed that the highest recorded UTS was 1262 ± 6 MPa, while the maximum elongation achieved was 16 ± 1%. Full article

The effect of various atmospheric parameters on the corrosion mechanism of press-hardened steel (PHS) coated with Al-Si (AS) was studied. Quantitative models of the composition of soluble and stable corrosion products were developed. A high chloride concentration led to a localized corrosion due to the presence of cracks in the coating. Increased corrosion resistance of silicon-rich Al₈Fe₂Si and AlFe at the expense of the Al₅Fe₂ phase with low silicon content was shown. Under low-chloride-deposition conditions, the coating exhibited good corrosion resistance and provided sufficient protection to the underlying steel. The formation of more local anodes and cathodes under conditions of lower relative humidity led to a reduction in the depth of corrosion pits in the steel substrate. Constant high relative humidity and sulphate deposits on the surface were critical for the acceleration of steel corrosion in coating cracks. Full article