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Severe tool wear significantly impacts both the tool life and machined surface quality when machining difficult-to-cut materials. Sufficient cooling and lubrication capacity provided by cutting fluid can effectively mitigate tool wear, whereas conventional flood cooling methods consume excessive amounts of cutting fluid, raising serious environmental concerns. The atomization mode of cutting fluid is accompanied by a minimum quantity of cutting fluid, and it has demonstrated exceptional cooling and lubrication performance for machining difficult-to-cut materials. In this work, various sustainable cooling conditions were employed, including dry cutting, high-pressure air cooling (HPAC), air atomization of cutting fluid (AACF), and ultrasonic atomization of cutting fluid (UACF). A series of experiments were implemented to investigate the tool wear behavior during the milling process of ultra-high-strength steel under different cooling conditions. The effects of tool wear evolution on the milling force, surface quality, and chip morphology were thoroughly analyzed. Results showed that micro-chipping, macro-chipping, and breakage were the primary wear evolution forms of the tool in HPAC condition, while the tools experienced abrasion, chipping, and breakage in the dry, AACF, and UACF processes. The occurrence of severe tool wear, leading to poor surface quality and chip tearing, was observed as the maximum flank wear width VBmax exceeded 0.2 mm. Compared to other cooling conditions, the UACF process exhibited the lowest tool wear and resultant milling force, as well as the best surface quality, attributed to the exceptional cooling and lubrication performance of droplets generated through ultrasonic atomization. [ABSTRACT FROM AUTHOR]

Ultra-high-strength steels have been considered an essential material for aviation components owing to their excellent mechanical properties and superior fatigue resistance. When machining these steels, severe tool wear frequently results in poor surface quality and low machining efficiency, which is intimately linked to the friction behavior at the tool–workpiece interface. To enhance the service life of tools, the adoption of efficient cooling methods is paramount. However, the understanding of friction behavior at the tool–workpiece interface under varying cooling conditions remains limited. In this work, both air atomization of cutting fluid (AACF) and ultrasonic atomization of cutting fluid (UACF) were employed, and their spray characteristic parameters, including droplet size distribution, droplet number density, and droplet velocity, were evaluated under different air pressures. Discontinuous sliding tests were conducted on the ultra-high-strength steel against cemented carbide and the effect of spray characteristic parameters on the adhesion friction coefficient was studied. The results reveal that ultrasonic atomization significantly improved the uniformity of droplet size distribution. An increase in air pressure resulted in an increase in both droplet number density and droplet velocity under both AACF and UACF conditions. Furthermore, the thickness of the liquid film was strongly dependent on the spray characteristic parameters. Additionally, UACF exhibited a reduction of 4.7% to 9.8% in adhesion friction coefficient compared to AACF. UACF provided the appropriate combination of spray characteristic parameters, causing an increased thickness of the liquid film, which subsequently exerted a positive impact on reducing the adhesion friction coefficient. [ABSTRACT FROM AUTHOR]

In recent years, with the rapid advancement of automotive lightweight technology, the mechanical clinching process between aluminum alloy and ultra-high-strength steel sheets has received extensive attention. However, the low ductility of ultra-high-strength steel sheets often results in conventional mechanical clinching processes producing joints that either fail to establish effective interlocks or cause the steel sheets to fracture. To address this issue, a novel mechanical clinching process is presented, called center-punching mechanical clinching (CPMC). This innovative process employs a method of punching, flanging, and bulging gradation to achieve the mechanical clinching of aluminum alloy and ultra-high-strength steel sheets in a single step. In order to determine the effects of different parameters on the quality and strength of the joint, an experimental study was carried out for various die depths and diameters based on the condition of constant punch size. Based on tensile and shear tests, the static strength and failure modes of CPMC joints were analyzed. The results indicated that the CPMC process significantly enhances the connectivity of joints for AA5052 aluminum alloy and DP980 ultra-high-strength steel. Optimal tensile and shear strengths of 1264 and 2249 N, respectively, were achieved at a die depth of 2.2 mm and a diameter of 10.4 mm. The CPMC process provides new ideas for the mechanical clinching of aluminum alloy and ultra-high-strength steels. [ABSTRACT FROM AUTHOR]

As high-strength and ultra-high-strength steels are widely used in all kinds of modern welded constructions, a lot of research is carried out to investigate the mechanical properties of the weldments of these steels, but there is little information on such important characteristics as their corrosion behaviour. This research focuses on the corrosion behaviour of the weld metal of the weldments of S906QL and S700MC steels. The weld metal was tested electrochemically in a 3.5% NaCl aqueous solution via a potentiodynamic scan to determine the corrosion rate and its dependence on the welding gap. No influence of the welding gap on the corrosion rate was found, but the experimental results suggested that the corrosion rate depended on the chemical composition of the filler material and the microstructure of the weld metal. [ABSTRACT FROM AUTHOR]

Automotive industries have seen increased demands for indirect hot-stamped pre-coated ultra-high-strength steel 22MnB5 due to its excellence. This paper presents the results of experimental investigation of the effect of Al-Si coating weights AS150 (150 g/m2) and AS80(80 g/m2) and spot-welding process parameters of indirect hot-stamped Usibor 1500 to further understand which coating weight perform better for automotive application. The resistance spot welding experiments were conducted with a medium-frequency direct control (MFDC) machine. The MFDC resistance spot-welding machine had two G20 electrodes with a 458 truncated cone designed with a diameter of 4.9 mm. The results show that the nugget diameter increases with increasing weld current, better spot-weld nugget diameter is recorded at weld currents above 7.5 kA for both AS80 and AS150 coating weights, and the weld current range decreases with increase in coating weight and with increase in the interdiffusion layer. Heat treatment dwell time has a remarkable influence on weld current range for both AS150 and AS80 coating weights. AS80 coating weight has a slightly higher weld current range of 2.1 KA than AS150 coating weight with weld current range of 1.8 KA. Tensile shear strength for AS150 and AS80 increases nonlinearly with increase in weld diameter. [ABSTRACT FROM AUTHOR]

The effect of varying rare earth element cerium (Ce) contents (0∼0.0792 wt%) on the austenite grain size of ultra-high-strength engineering steel has been studied under different austenitization conditions (holding time of 5∼360 min at temperatures ranging from 900 °C to 1200 °C). Results show that the addition of Ce can refine grains, and the refining effect becomes more pronounced as the Ce content increases. The inhibition of austenite grain growth by fine Ce-containing inclusions during the holding process has been observed through laser scanning confocal microscopy (LSCM) and field-emission scanning electron microscope (FE-SEM) equipped with an energy dispersive spectrometer (EDS), and the pinning effect of Ce-containing inclusions on the austenite grain boundaries is the primary factor for the refinement of austenite grains. Besides, theoretical calculations have shown that the lattice misfits between the (100) plane of CeP and the (111) plane of the γ-phase is 7.83%, indicating that CeP could serve as a heterogeneous nucleation core for the γ-phase and was moderately effective. Furthermore, the uniformization effect of Ce on the grains is more prominent than the refining effect at the holding temperatures of 1000 °C and 1200 °C.

Featured Application: Numerical results to determine the fire resistance of ultra-high-strength steel using different heating rates. Ultra-High-Strength Steel (UHSS) offers several advantages over normal carbon steel, promoting exceptional strength, reducing self-weight, improving fire resistance, enhancing durability, and reducing material consumption. These advantages result in cost savings and sustainable engineering construction. The 3D numerical model is based on Geometrical and Materially Nonlinear Imperfection Analysis (GMNIA) and determines the fire resistance of different cross-section columns. The model is validated with experimental tests, with a maximum relative error of 11%. A parametric analysis is presented, based on 252 simulations, assuming three heating rates, two different cross-sections, two different thicknesses, three lengths, and seven load levels. The fire resistance depends on the heating rate, but the critical temperature is almost equal and independent of the heating rate, if one assumes implicit creep in the constitutive material model. The fire resistance decreases with the load level, as expected. The thickness effect of the hollow section is almost negligible in the fire resistance of UHSS columns. The fire resistance decreases more in higher load levels for slender columns. Columns with Circular Hollow Sections (CHSs) generally show higher fire resistance than hybrid columns in longer columns, but the hybrid columns are subject to much higher loads. New design formulas are presented for the critical temperature of UHSS columns, depending on the load level and slenderness of two different cross-sections. [ABSTRACT FROM AUTHOR]

The investigation was motivated by the widespread use of ultra-high-strength steels in cable-stayed bridges and armor steel plates. These structures are exposed to elevated risks from extreme loads, including vehicle impacts and blast loadings. The study aimed to investigate the mechanical properties of 1770 steel for applications in extreme conditions. The strain rates and temperatures ranged from 0.00095 s−1 to 1303 s−1 and 20 °C–900 °C, respectively. Tensile tests were conducted at various strain rates using a Split Hopkinson Tension Bar for dynamic testing and a universal testing machine for quasi-static testing at room temperature (20 °C). Quasi-static tension at elevated temperatures was carried out using an electromechanical testing machine coupled with a heating device. A uniform temperature distribution inside the specimens was ensured by a 30-min holding time. The results of the tensile test indicate that the 1770 steel displays notable thermal softening effects and strain rate hardening that differ from mild steel. The standard Johnson-Cook (J-C) model was found to be inadequate in accurately predicting the strain rate effects of this specimen. To address this issue, the strain rate parameter C in the standard model was adjusted to create the modified J-C model, which demonstrated good agreement with the experimental results. To further demonstrate the modified model and the identified parameters, drop hammer impact tests and corresponding numerical simulations were conducted. The impact test and numerical simulation results showed good agreement in terms of deformation and contact force. The mechanical properties and the constitutive models can serve as a design reference for the ultra-high-strength 1770 steel in extreme envirnments, as well as a theoretical basis for fitting material models of similar ultra-high-strength steels.

The powder metallurgy forming of ultra-high-strength steel (UHSS) G33 and its combination with high toughness titanium alloy Ti–7Al–1Mo-0.5V-0.1C were achieved through the application of Ta/Ni dual-interlayer in a hot-press sintering diffusion bonding (HPSDB) process, conducted at 970 °C, 50 MPa, and a duration of 3 h. The resulting bonding strength proved excellent, with a high shear strength of 485.5 MPa. The interface structure, precipitated phases, and element diffusion behavior were investigated through scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS). Notably, no intermetallic compounds (IMCs) precipitated in the Ti–Ta diffusion zone or the Ni-G33 diffusion zone, underscoring the effectiveness of the metallurgical bond. Selected area electron diffraction (SAED) and phase diagram calculation were employed to elucidate the mechanisms contributing to the robust bonding strength of the joint with Ta/Ni dual-interlayer. The results revealed that the diffusion zone primarily comprised NiTa2, Ni2Ta, and Ni3Ta with high shear modulus and excellent ductility. Near the bonding interface, Ni3Ta grains exhibited approximate low-angle torsional grain boundaries, effectively impeding dislocation movement. Moreover, the bonding strength was further enhanced by the precipitation of nanoscale tetragonal-Ta dispersed within the Ni2Ta matrix.

In recent years, with the rapid advancement of automotive lightweight technology, the mechanical clinching process between aluminum alloy and ultra-high-strength steel sheets has received extensive attention. However, the low ductility of ultra-high-strength steel sheets often results in conventional mechanical clinching processes producing joints that either fail to establish effective interlocks or cause the steel sheets to fracture. To address this issue, a novel mechanical clinching process is presented, called center-punching mechanical clinching (CPMC). This innovative process employs a method of punching, flanging, and bulging gradation to achieve the mechanical clinching of aluminum alloy and ultra-high-strength steel sheets in a single step. In order to determine the effects of different parameters on the quality and strength of the joint, an experimental study was carried out for various die depths and diameters based on the condition of constant punch size. Based on tensile and shear tests, the static strength and failure modes of CPMC joints were analyzed. The results indicated that the CPMC process significantly enhances the connectivity of joints for AA5052 aluminum alloy and DP980 ultra-high-strength steel. Optimal tensile and shear strengths of 1264 and 2249 N, respectively, were achieved at a die depth of 2.2 mm and a diameter of 10.4 mm. The CPMC process provides new ideas for the mechanical clinching of aluminum alloy and ultra-high-strength steels.

Ultra-high-strength steels have been considered an essential material for aviation components owing to their excellent mechanical properties and superior fatigue resistance. When machining these steels, severe tool wear frequently results in poor surface quality and low machining efficiency, which is intimately linked to the friction behavior at the tool–workpiece interface. To enhance the service life of tools, the adoption of efficient cooling methods is paramount. However, the understanding of friction behavior at the tool–workpiece interface under varying cooling conditions remains limited. In this work, both air atomization of cutting fluid (AACF) and ultrasonic atomization of cutting fluid (UACF) were employed, and their spray characteristic parameters, including droplet size distribution, droplet number density, and droplet velocity, were evaluated under different air pressures. Discontinuous sliding tests were conducted on the ultra-high-strength steel against cemented carbide and the effect of spray characteristic parameters on the adhesion friction coefficient was studied. The results reveal that ultrasonic atomization significantly improved the uniformity of droplet size distribution. An increase in air pressure resulted in an increase in both droplet number density and droplet velocity under both AACF and UACF conditions. Furthermore, the thickness of the liquid film was strongly dependent on the spray characteristic parameters. Additionally, UACF exhibited a reduction of 4.7% to 9.8% in adhesion friction coefficient compared to AACF. UACF provided the appropriate combination of spray characteristic parameters, causing an increased thickness of the liquid film, which subsequently exerted a positive impact on reducing the adhesion friction coefficient.

As high-strength and ultra-high-strength steels are widely used in all kinds of modern welded constructions, a lot of research is carried out to investigate the mechanical properties of the weldments of these steels, but there is little information on such important characteristics as their corrosion behaviour. This research focuses on the corrosion behaviour of the weld metal of the weldments of S906QL and S700MC steels. The weld metal was tested electrochemically in a 3.5% NaCl aqueous solution via a potentiodynamic scan to determine the corrosion rate and its dependence on the welding gap. No influence of the welding gap on the corrosion rate was found, but the experimental results suggested that the corrosion rate depended on the chemical composition of the filler material and the microstructure of the weld metal.

The wear tests are conducted on bending punches deposited with PVD CrN and AlTiCrN coatings using the newly proposed progressive die. Then, the surface quality of the formed product is characterized through the surface roughness measurement after forming of TRIP1180 steel sheets. The correlation between the tool wear, in terms of wear depth and roughness and the product surface roughness can be quantitatively analyzed. The results show that the roughness remains comparable to that of the as-received surface before failure occurs, which represents smooth product surface without severe scratches and defects. While micro scratches on the punch surface have no effect on the quality of the product surface, severe fretting wear on the punch surface leads to a deterioration in the surface quality. Once initiated in the stamping process, the wear progresses exponentially within short time. The wear is also characterized as less than the coating thickness, but it results in complete removal of the coating layer. The partially worn punch plows the product surface, causing surface scratches with grooves and ridges, resulting in the roughness of 1.0 μm. In contrast, the surface with completely damaged coatings is extremely rough, with the roughness of 2.0 μm. This study presents the efficient method to evaluate the tool wear progression by indirectly measuring the product surface quality with reliably high precision. [ABSTRACT FROM AUTHOR]

To solve the problem that twist springback of thin-walled component with asymmetric complex section cannot meet the high-precision quality requirement, FEA (finite element analysis) model of multi-pass roll forming (RF) process is established. Different material constitutive models are verified with RF tests, and the developed FEA model with variable elastic modulus and Yld2000 yield criterion is carried out to predict twist springback accurately. The mechanism of twist springback is studied by numerical simulations that twist springback results from the torque produced with the uneven residual longitudinal stress distribution due to the asymmetric forming for asymmetric section. The effect of different forming strategies on twist springback is discussed that developed UDT-type (USTB-Durable T) method with constant-arc-length method can effectively control twist springback. Orthogonal test is designed to study the effect of process parameters on twist springback. Results indicate that forming stands has the greatest impact on twist angle, followed by roll diameter and inter-distance, and roll gap has the smallest impact. The developed design and forming parameters are applied to control twist springback of asymmetric complex-section product. The actual product is deformed in industrial production line with the twist angle of less than 2 ∘ / m to meet high-precise quality requirement. [ABSTRACT FROM AUTHOR]

Microstructural transformations and mechanical properties of weld metals obtained by gas metal arc welding (GMAW) of S960QL quenched and tempered ultra-high-strength steels (UHSS) with 15 mm thickness were investigated. Welded joints were acquired by using undermatching filler metal. Selected weaving and non-weaving multi-pass procedures have influenced weld metal heat inputs and number of passes. Mechanical properties of the weld metals were characterized by Charpy, hardness, and tensile tests, and these results were correlated with microstructural characterizations using an optical microscope and scanning electron microscope. Acicular ferrite is predominant in both of the weld metals, and microstructure of the weld metal obtained by weaving multi-pass has coarser grain structure. Fine lath martensite formation in non-weaving joint's weld metal with low heat input was more intense than weaving joint's weld metal with high heat input. Lath martensite acquired due to rapid cooling in non-weaving joint's weld metal provided adequate strength properties. Although toughness values were low, they have met relevant standard requirements. [ABSTRACT FROM AUTHOR]

The effects of a sheared edge and overlap length on the reduction in the tensile fatigue limit before and after hydrogen embrittlement of resistance spot-welded ultra-high-strength steel sheets were investigated. Ultra-high-strength steel sheets with sheared and laser-cut edges were subjected to resistance spot welding followed by hydrogen embrittlement via cathodic hydrogen charging and subjected to static tensile shear and fatigue tests. The distance between the resistance spot weld and the sheared and laser-cut edges was changed by changing the overlap length, and the influence of the weld position was investigated. In the tensile shear test, the maximum load decreased with decreasing overlap length and the maximum load decreased with hydrogen embrittlement, but the effect of hydrogen embrittlement was smaller than that in the fatigue test. In the fatigue test, the fatigue mode changed from the width direction to the sheared edge direction with the increase in the repeated load. Even if the overlap length was reduced, the fracture changed to the sheared edge direction. In the specimens with sheared edges, the effect of fatigue limit reduction due to hydrogen embrittlement was greater than in the specimens with laser surfaces. In particular, the effect was greatest when the fatigue mode was changed via hydrogen embrittlement. [ABSTRACT FROM AUTHOR]

Ultra-High-Strength Steel (UHSS) offers several advantages over normal carbon steel, promoting exceptional strength, reducing self-weight, improving fire resistance, enhancing durability, and reducing material consumption. These advantages result in cost savings and sustainable engineering construction. The 3D numerical model is based on Geometrical and Materially Nonlinear Imperfection Analysis (GMNIA) and determines the fire resistance of different cross-section columns. The model is validated with experimental tests, with a maximum relative error of 11%. A parametric analysis is presented, based on 252 simulations, assuming three heating rates, two different cross-sections, two different thicknesses, three lengths, and seven load levels. The fire resistance depends on the heating rate, but the critical temperature is almost equal and independent of the heating rate, if one assumes implicit creep in the constitutive material model. The fire resistance decreases with the load level, as expected. The thickness effect of the hollow section is almost negligible in the fire resistance of UHSS columns. The fire resistance decreases more in higher load levels for slender columns. Columns with Circular Hollow Sections (CHSs) generally show higher fire resistance than hybrid columns in longer columns, but the hybrid columns are subject to much higher loads. New design formulas are presented for the critical temperature of UHSS columns, depending on the load level and slenderness of two different cross-sections.

Due to the different composition of cutting fluid will affect the failure stress of the material in the shear deformation area, so the rational use of cutting fluid can optimize the cutting force and the surface integrity of parts. Four kinds of hat-shaped samples with shear band widths were designed. The effects of different cutting fluids on mechanical properties of ultrahigh strength steel at different shear strain rates were studied in this paper by SHPB, and verified by cutting experiments. The stress–strain curve, shear failure stress, fracture morphology and cutting force were studied systematically. Experimental results show that the stress remains stable for a period of time with the increases of strain after strain hardening at γs= 10.03 × 104 1/s. All shear fracture is ductile fracture and at γs

Corrosion is a major factor that inhibits the longevity of automotive components especially in countries where significant amount of salt is sprayed on highways during winter. In this paper the effect of cosmetic, perforation and red pitting corrosion on indirect hot-stamped Ultra-High-Strength-Steel UsiborR 1500 were investigated using two different Al–Si coating weights AS80 and AS150. The experiments were conducted in a cyclic corrosion chamber over 12 weeks. The results show that AS150 and AS80 exhibited similar excellent perforation corrosion resistance compared to 22MnB5 due to highly protective corrosion products; Both AS80 and AS150 show maximum depth of corrosion attack less than 200 µm compared to 22MnB5 of 900 µm. AS150 displayed better resistance to cosmetic corrosion than AS80. AS80 coating shows higher resistance to red rust pitting corrosion than AS150. E-coat thickness of 15 µm results in higher resistance to red-pitting corrosion than lower e-coat of 8 µm. [ABSTRACT FROM AUTHOR]

The torsion spring of a carrier-based aircraft landing gear is a key component, which is normally manufactured out of AerMet100 ultra-high-strength steel. The takeoff and landing performance is greatly influenced by its bearing capacity and structural durability. To carry out the structure anti-fatigue design, it is necessary to investigate the influence of the spring structure features on its fatigue life, based on which the strain fatigue analysis and parameter optimization design of the torsion spring are executed. Through the finite element analysis conducted with ABAQUS, it was determined that there exists serious stress concentration in the relief groove. Based on the theory of strain fatigue, the fatigue life of the torsion spring was obtained, and the fracture position and lifecycle were consistent with the test results. A structure optimization platform based on a parametric method was established. Samples were selected through the DOE (design of experiment), and a surrogate model was established based on RBF (radial basis functions), followed by optimization using MIGA (multi-island genetic algorithms). With the parameter optimization of the relief groove, the structure was reconstituted and reanalyzed. From the simulation results, the peak strain was reduced by 30.7%, while the fatigue life was increased by 86.2% under the same loads and constraints. Moreover, laboratory tests were performed on the torsion spring after reconstruction, which showed that the fatigue life increases by 85.6% after optimization. The method presented in this paper can provide theoretical support and technical guidance for the application and structural optimization of ultra-high-strength steel structures. [ABSTRACT FROM AUTHOR]

This study investigates four different joints under fluctuating tension cycles in the low-cycle fatigue (LCF) regime. The selected material was S960 ultra-high-strength steel, manufactured via the direct-quenching manufacturing route. Based on typical engineering solutions, the following joints were chosen to be investigated in the experimental tests: metal active gas and laser welded butt joints, fillet-welded load-carrying cruciform joints, and non-load-carrying double-sided transverse attachment joints. To evaluate the improvement gained by the post-weld treatment, both as-welded condition and high-frequency mechanical impact treated joints from all joint types were tested. In addition to the experimental tests, some of the joints were analyzed using finite element (FE) models, with the aim of comparing the accuracy of the different fatigue assessment methods and the effect of the toe radius on the computational fatigue lives. The results obtained from the experimental tests and FE analyses were compared with each other and with IIW recommendations using the nominal stress, structural hot-spot (HS) stress, effective notch stress (ENS) and 4R methods. Based on the results, the structural HS and 4R methods are very suitable for the fatigue strength assessment in the LCF regime. With the nominal stress method, the computational fatigue lives remained uniformly conservative, while with the ENS method, the results varied depending on the calculation method. Using the value r = 1 mm as the weld toe rounding is justified in the FE analysis. [ABSTRACT FROM AUTHOR]

Ultra-high-strength steel (UHSS) is a complex and sophisticated material that allows the development of products with reduced weight but increased strength and can assist, for example, in the automotive industry, saving fuel in vehicles and decreasing greenhouse gas emissions. Welding UHSS has a certain complexity, mainly due to the higher alloys and heat treatments involved, which can result in a microstructure with higher sensitivity to welding. The primary purpose of the current work was to select the best parameters of the gas metal arc welding (GMAW) for welding the S960 material based on prediction methods. To achieve the expected results, a finite element analysis (FEA) was used to simulate and evaluate the results. It was found that the welding parameters and, consequently, the heat input derived from the process greatly affected the UHSS microstructure. Using FEA and estimating the extension of the heat-affected zone (HAZ), the peak temperature, and even the effect of distortion and shrinkage was possible. With an increase in the heat input of 8.4 kJ/cm, the estimated cooling rate was around 70 °C/s. The presence of a softening area in the coarse grain heat-affected zone (CGHAZ) of welded joints was identified. These results led to an increase in the carbon content (3.4%) compared to the base metal. These results could help predict behaviors or microstructures based on a few changes in the welding parameters. [ABSTRACT FROM AUTHOR]

Based on the indirect hot-stamping test system, the effect of pre-forming on the microstructure evolution (grain size, dislocation density, martensite phase transformation) and mechanical properties of the blank in indirect hot stamping is systematically studied using ultra-high-strength steel 22MnB5. It is found that the average austenite grain size slightly decreases with the increase in pre-forming. After quenching, the martensite also becomes finer and more uniformly distributed. Although the dislocation density after quenching slightly decreases with the increase in pre-forming, the overall mechanical properties of the quenched blank are not greatly affected by pre-forming under the combined effect of the grain size and dislocation density. Then, this paper discusses the effect of the pre-forming volume on part formability in indirect hot stamping by manufacturing a typical beam part. According to the numerical simulations and experimental results, when the pre-forming volume increases from 30% to 90%, the maximum thickness thinning rate of the beam part decreases from 30.1% to 19.1%, and the final beam part has better formability and more uniform thickness distribution results when the pre-forming volume is 90%. [ABSTRACT FROM AUTHOR]

The surface integrity of ultra-high-strength steel has a significant influence on service performance, and cutting fluid plays an important role in maintaining surface integrity in production. In this paper, the surface integrity of ultra-high-strength steel 45CrNiMoVA was investigated under three cutting fluids: HY-103 (micro-emulsion), TRIM E709 (emulsion), and Vasco 7000 (micro-emulsion) from the aspects of cutting force, surface morphology, residual stress, micro hardness, microstructure, etc. The results showed that the changing trend of the cutting forces in three directions is HY-103 > Vasco 7000 > TRIM E709. The TRIM E709 contains the maximum lubricants, which reduce cutting force and Sa roughness, while the Vasco 7000 contains the minimum corrosive elements, which results in the least pitting. Both tangential and axial stresses under cutting fluid are tensile stresses. TRIM E709 and Vasco 7000 are reduced axially by 4.45% and 7.60% relative to HY-103, respectively. The grain refinement layer depths of HY-103, TRIM E709, and Vasco 7000 are 9 μm, 4 μm, and 8 μm, respectively, and TRIM E709 can induce recrystallized grains to grow along {001} of the sample cross section, which results from the lowest cooling rate. This work may provide an innovative control strategy for cutting fluid to improve surface integrity and service performance. [ABSTRACT FROM AUTHOR]

Mechanical material behaviour of ultra‐high‐strength structural steel S960QL in case of fire Due to their good strength‐to‐weight ratio, high‐ and ultra‐high‐strength structural steels are increasingly used in multi‐storey and industrial buildings in the form of plate material, pipes and sections. For structures with fire protection requirements, however, the use of high‐strength structural steels is currently inhibited by a lack of knowledge regarding temperature‐ and rate‐dependent material behaviour. Comprehensive knowledge of the material behaviour is a basis for a realistic modeling of the load‐bearing behaviour of steel structures. Within the framework of the FOSTA‐research project P 1502, systematic investigations of the structural material behaviour of ultra‐high‐strength steels are carried out in terms of an extensive tensile test program. The aim of the project is to characterise the mechanical material behaviour during all phases of a fire, including the cooling phase of natural fire scenarios. The paper addresses the material behaviour of quenched and tempered steel S960QL in fire. Steady state and transient state tensile test results are presented and used for the characterisation of the material behaviour during the heating phase of a fire and the comparison to common material models. In addition, the results of natural fire tests for the evaluation of the material behaviour in the cooling phase and the residual behaviour are presented. [ABSTRACT FROM AUTHOR]

The effects of a sheared edge and overlap length on the reduction in the tensile fatigue limit before and after hydrogen embrittlement of resistance spot-welded ultra-high-strength steel sheets were investigated. Ultra-high-strength steel sheets with sheared and laser-cut edges were subjected to resistance spot welding followed by hydrogen embrittlement via cathodic hydrogen charging and subjected to static tensile shear and fatigue tests. The distance between the resistance spot weld and the sheared and laser-cut edges was changed by changing the overlap length, and the influence of the weld position was investigated. In the tensile shear test, the maximum load decreased with decreasing overlap length and the maximum load decreased with hydrogen embrittlement, but the effect of hydrogen embrittlement was smaller than that in the fatigue test. In the fatigue test, the fatigue mode changed from the width direction to the sheared edge direction with the increase in the repeated load. Even if the overlap length was reduced, the fracture changed to the sheared edge direction. In the specimens with sheared edges, the effect of fatigue limit reduction due to hydrogen embrittlement was greater than in the specimens with laser surfaces. In particular, the effect was greatest when the fatigue mode was changed via hydrogen embrittlement.

In this paper, a flow stress model of ultra-high-strength steel 1700MS was developed for laser welding. The model considered the material behaviors of strain hardening, temperature sensitivity, and annealing. The prediction yield strengths had good accordance with the experimental results. The effects of the three behaviors on welding stresses were studied through four calculation cases. The results showed that the behaviors should all be taken into account and there was a reasonable agreement between the predicted and measured stresses. In addition, the stress evolution process of each case was analyzed through the temperature-dependent curves of variables. The variables contained the accumulated plastic strain, deviatoric stress tensor, plastic strain, trial stress, and actual stress used in the stress updating process. It was found the behaviors could change the yield strength used in each case. The temperature sensitivity and annealing increased the possibility of plastic deformation, while the strain hardening decreased and thus gave rise to different stress updating rules.

The effect of tempforming on the strength and fracture toughness of 0.4%C-2%Si-1%Cr-1%Mo-VNb steel was examined. Plate rolling followed by tempering at the same temperature of 600 °C increases yield stress by 25% and the Charpy V-notch impact energy by a factor of ~10. Increasing rolling reduction leads to the reorientation and elongation of grains toward the rolling direction (RD) and the development of a strong {001} <110> (rotated cube) texture component that highly enhances fracture toughness. A lamellar structure with a spacing of 72 nm between boundaries and a lattice dislocation density of ~1015 m−2 evolves after tempforming at 600 °C with a total strain of 1.4. Two types of delamination were found, attributed to crack branching and the propagation of secondary cracks along the rolling plane perpendicular to the propagation direction of the primary crack. Delamination toughness is associated with the nucleation of secondary cracks in RD and their propagation over a large distance. The critical condition for delamination toughness is the propagation of primary cracks by the ductile fracture mechanism and the propagation of secondary cracks by the brittle quasi-cleavage mechanism. [ABSTRACT FROM AUTHOR]

The body of an automobile must be lightweight and crash-worthy. The use of advanced high-strength steel reduces the manufacturing cost for automobile body and improves energy absorption during an impact. However, increasing the strength of the steel sheet results in a variety of issues related to tool wear due to a higher forming pressure in the sheets than that in the conventional low-strength steel sheets. Therefore, systematic wear experimentation and evaluation procedure are required to quantify the extent of tool wear during the press forming process. This study aims to develop a methodology for quantifying the wear of a sheet metal forming tool using experimental databases. A progressive die set is designed to conduct the wear test systematically, saving both time and money. The testing machine is capable of testing four different types of punches simultaneously under a variety of tooling and process parameters, such as different tool materials, punch shapes, and coating conditions. The wear depth, roughness, and surface imaging of the tool are all used to establish quantitative evaluation methods for tool wear in the sheet metal forming process. Consequently, we found that punches with small radius of curvature wear rapidly, while tools and coatings with a high hardness have high resistance against wearing. Additionally, by referring to the measured wear database, it is confirmed that analyzing the reasonable tool wear characteristics is appropriate. The proposed testing method can be used to obtain reliable results for quantitatively and qualitatively evaluating the wear lifetime of various tool materials. [ABSTRACT FROM AUTHOR]

The advanced lamellar microstructure significantly improves the toughness of Cu-bearing ultra-high strength steel by delamination toughening (yield strength: 1370 MPa, impact toughness at −40 °C: 60 J). The lamellar microstructure affects the microstructure evolution of heat-affected zone (HAZ), resulting in separate distributions of lath martensite and granular bainite in the complete austenitizing zone and the formation of cluster fresh martensite in the partial austenitizing zone. The grain refinement and decrease in dislocation density are predominant features, especially for the complete austenitizing zone, where the grain is refined to 4.33 μm, and dislocation density is decreased by 27%. With the degree of austenitizing increase, the dissolution of Cu-rich precipitates (CRPs) aggravates during welding. A small fraction of CRPs in the complete austenitizing zone implies the onset of reprecipitation of CRPs. The reason for softening in HAZ is attributed to a combined effect of granular bainite forming, dislocation density decreasing, and CRPs dissolving. After PWTH, large numbered reprecipitation of coherent CRPs occurs, contributing to the hardness recovery of HAZ. Meanwhile, due to the high density of dislocation of lamellar microstructure inherited by partial austenitizing zone, coarsening of coherent CRPs is easy to occur, and various incoherent structures are observed. [ABSTRACT FROM AUTHOR]

The changes in the magnetization properties of high-strength steel and ultra-high-strength steel sheets are investigated, and then the sheared edges and the scattering behavior of slivers in shearing of the ultra-high-strength steel sheets are observed. The maximum magnetic flux density of the magnetized sheet is increased with the increasing tensile strength of the sheet. The maximum magnetic flux density in the magnetized blanks decreases, whereas the density in the demagnetized blanks increases. In the sheared edges, the ratio of the fracture surface becomes larger with the increasing tensile strength of the steel sheet. In shearing, the shearing slivers are observed at the time of crack penetration and at the time of punch rise. The mass of the slivers generated from the blank in shearing increases with the increasing tensile strength of the steel sheet. Two-thirds of the generated shearing slivers stick to the blank in the magnetized blank, whereas two-thirds of the slivers in the blank without magnetization scatter to the outside of the die. [ABSTRACT FROM AUTHOR]

The torsion spring of a carrier-based aircraft landing gear is a key component, which is normally manufactured out of AerMet100 ultra-high-strength steel. The takeoff and landing performance is greatly influenced by its bearing capacity and structural durability. To carry out the structure anti-fatigue design, it is necessary to investigate the influence of the spring structure features on its fatigue life, based on which the strain fatigue analysis and parameter optimization design of the torsion spring are executed. Through the finite element analysis conducted with ABAQUS, it was determined that there exists serious stress concentration in the relief groove. Based on the theory of strain fatigue, the fatigue life of the torsion spring was obtained, and the fracture position and lifecycle were consistent with the test results. A structure optimization platform based on a parametric method was established. Samples were selected through the DOE (design of experiment), and a surrogate model was established based on RBF (radial basis functions), followed by optimization using MIGA (multi-island genetic algorithms). With the parameter optimization of the relief groove, the structure was reconstituted and reanalyzed. From the simulation results, the peak strain was reduced by 30.7%, while the fatigue life was increased by 86.2% under the same loads and constraints. Moreover, laboratory tests were performed on the torsion spring after reconstruction, which showed that the fatigue life increases by 85.6% after optimization. The method presented in this paper can provide theoretical support and technical guidance for the application and structural optimization of ultra-high-strength steel structures.

The spatiotemporal phase transformation during hot stamping would considerably effect the microstructure and mechanical properties of steels. In order to manufacture hot-stamping components of ultra-high-strength steel with tailored mechanical properties, the effect of the quenching time and the temperature of the die on the phase-transformation characteristics and mechanical property of ultra-high-strength steel was deeply studied. A finite element (FE) model coupled with a thermomechanical phase was employed to perform a succession of simulations for hot stamping corresponding to different quenching times and the temperatures of die, and the corresponding hot stamping experiments were performed. The 3D mapping surfaces of the temperature; quenching time; and three microstructures, namely austenite, bainite, and martensite, were constructed, and the mapping relationships in such surfaces were further explained by microstructural observations. Subsequently, based on the test results of the mechanical properties, the relationship curves of hardness and tensile strength, hardness, and elongation at break were fitted respectively, and then the 3D mapping surfaces were constructed for hardness, tensile strength, and elongation at break, which varied with the temperature die and quenching time. Finally, the quenching parameters of the automobile B-pillar were designed according to the constructed mapping relationship, and the hot-stamping FE simulation of the automobile B-pillar was developed. The result shows that those constructed mapping surfaces are helpful for adjusting the local mechanical property of the steels by designing the parameters. [ABSTRACT FROM AUTHOR]

Ultra-high-strength steel significantly contributes to lightweight design. However, it is essential that excellent mechanical properties are maintained after the applied welding process in terms of structural integrity, strength and ductility. This study investigates the microstructural and mechanical properties of metal active gas-welded X-joints of 20-mm-thick S1100 thermomechanically rolled plates with undermatched alform 960 L-MC filler metal. Welding was carried out fully automated in order to obtain uniform properties of the welds for different heat inputs in flat (PA) and vertical up (PF) positions. The mechanical properties of weldments were characterized by hardness, impact and tensile tests and complemented by microstructural analysis using optical and scanning electron microscopy. Different heat inputs due to altered welding positions caused changes in prior austenite grain size and orientation in the weld metal (WM) microstructure which is presented and discussed in detail. The microstructures of the WM consist of ferrite, while the heat-affected zone (HAZ), which was exposed to several thermal cycles, is dominated by bainite and martensite-austenite (M/A) constituents. While in the HAZ the top layer shows fresh martensite of high hardness, the HAZ of filler and root passes was tempered showing lower hardness values. Transverse tensile tests always failed in the weld metal not fulfilling base materials strength requirements. Although the weld metal microstructure consisted of acicular ferrite in PA and PF position, PF weld has lower toughness. [ABSTRACT FROM AUTHOR]

Ultra-high-strength steel (UHSS) is a complex and sophisticated material that allows the development of products with reduced weight but increased strength and can assist, for example, in the automotive industry, saving fuel in vehicles and decreasing greenhouse gas emissions. Welding UHSS has a certain complexity, mainly due to the higher alloys and heat treatments involved, which can result in a microstructure with higher sensitivity to welding. The primary purpose of the current work was to select the best parameters of the gas metal arc welding (GMAW) for welding the S960 material based on prediction methods. To achieve the expected results, a finite element analysis (FEA) was used to simulate and evaluate the results. It was found that the welding parameters and, consequently, the heat input derived from the process greatly affected the UHSS microstructure. Using FEA and estimating the extension of the heat-affected zone (HAZ), the peak temperature, and even the effect of distortion and shrinkage was possible. With an increase in the heat input of 8.4 kJ/cm, the estimated cooling rate was around 70 °C/s. The presence of a softening area in the coarse grain heat-affected zone (CGHAZ) of welded joints was identified. These results led to an increase in the carbon content (3.4%) compared to the base metal. These results could help predict behaviors or microstructures based on a few changes in the welding parameters.

Despite significant advances in metallurgy and the potential to create high and ultra-high-strength steel, all international specifications for steel design provide little information about the limits of slenderness for high-strength steel sections (HSSs) and don't provide anything about the design of ultra-high-strength steel sections (UHSSs). The current international steel codes such as AISC 360-16 and EC3 can be applied only to steel grades up to S690 and S460, respectively, according to their limitations. These approaches for normal-strength steel are used for HSSs and UHSSs without extensive studies to determine their accuracy in these cases. Therefore, it is one of the main objectives of this study. The behavior of high and ultra-high-strength steel circular hollow sections under axial compression load is studied in this research. Sixteen nonlinear finite element (FE) models were generated to replicate stub column tests that were experimentally tested by others in previous research. Hence, a parametric study was conducted using forty FE models developed to investigate the local buckling behavior under various slenderness ratios comprehensively. The developed models covered slenderness ratios ranging from 20 to 1226 and steel grades S460 and S1100 with yield stress equal to 460 MPa and 1152 MPa, respectively. The FE results were combined with 105 previously collected experimental results to assess the applicability of existing codified design methodologies in the Euro code and the North American codes of cold-formed circular hollow sections (CHSs). Based on the results of this study, new cross-section slenderness limits and new design equations for more efficient simple designs were presented for circular hollow sections of HSSs and UHSSs and compared with the results of experimental tests and FE models. [ABSTRACT FROM AUTHOR]

• The influence of stress state on fracture initiation is studied for Q960 UHSS. • Stress-state coupled damage variable is proposed for undergoing large deformation. • The cold bendability of Q960 UHSS is assessed with H-C fracture initiation locus. Ultra-high-strength structural steels are gaining popularity in civil engineering due to their exceptional strength-to-weight ratio. However, their inherent lower ductility compared to normal strength structural steel warrants particular attention and further investigation for safety assessment. In this study, a total of 24 specimens made of Q960 ultra-high-strength steel (UHSS), including 21 tensile specimens with various stress states and 3 three-point bending specimens, were tested to assess its mechanical behavior undergoing large deformation. Subsequently, the impact of stress state on plasticity, damage evolution and fracture initiation was analyzed and characterized. The Bai-Wierzbicki yield surface with a deviatoric associated flow rule was identified to characterize the complex plasticity behavior of Q960 UHSS. With respect to damage evolution, the stress-state dependent fracture energy G f was proposed in this study to describe the damage softening behavior. The uncoupled Hosford-Coulomb locus with a non-linear weight function was utilized to predict the fracture initiation behavior. Finally, the utilized stress-state dependent constitutive models for Q960 UHSS, with the calibrated parameters and user-defined material subroutine VUMAT, were successfully verified through the three-point bending test with good agreement. Moreover, based on a parametric analysis of radius-to-thickness ratio, the bendability design regarding the minimum radius-to-thickness ratio of Q960 UHSS was proposed. [ABSTRACT FROM AUTHOR]

Based on the indirect hot-stamping test system, the effect of pre-forming on the microstructure evolution (grain size, dislocation density, martensite phase transformation) and mechanical properties of the blank in indirect hot stamping is systematically studied using ultra-high-strength steel 22MnB5. It is found that the average austenite grain size slightly decreases with the increase in pre-forming. After quenching, the martensite also becomes finer and more uniformly distributed. Although the dislocation density after quenching slightly decreases with the increase in pre-forming, the overall mechanical properties of the quenched blank are not greatly affected by pre-forming under the combined effect of the grain size and dislocation density. Then, this paper discusses the effect of the pre-forming volume on part formability in indirect hot stamping by manufacturing a typical beam part. According to the numerical simulations and experimental results, when the pre-forming volume increases from 30% to 90%, the maximum thickness thinning rate of the beam part decreases from 30.1% to 19.1%, and the final beam part has better formability and more uniform thickness distribution results when the pre-forming volume is 90%.

• Fracture toughness of 1.7 GPa-grade martensitic low-alloy steel sheet. • Combined testing procedure the ASTM-E561 and -E399 standards. • Unraveling the crack-closure effect on the threshold for planar crack propagation. • The intrinsic toughness without closure effects was invariable with any thickness. A novel testing method was developed to evaluate the fracture toughness of ultra-high-strength (UHS) steel sheets. The study also examined the influence of specimen thickness, utilizing specimens of varying thicknesses (1.6 ∼ 10 mm). While the toughness of all specimens was determined by the threshold for planar crack propagation originating from the fatigue pre-crack-front near the mid-thickness zone, the apparent fracture toughness was dependent on specimen thickness. This phenomenon was attributed to the crack-closure effects experienced during the pre-cracking process. In-depth analyses revealed that, in the absence of crack-closure effects, the intrinsic toughness remained unaffected by changes in specimen thickness. [ABSTRACT FROM AUTHOR]

The surface integrity of ultra-high-strength steel has a significant influence on service performance, and cutting fluid plays an important role in maintaining surface integrity in production. In this paper, the surface integrity of ultra-high-strength steel 45CrNiMoVA was investigated under three cutting fluids: HY-103 (micro-emulsion), TRIM E709 (emulsion), and Vasco 7000 (micro-emulsion) from the aspects of cutting force, surface morphology, residual stress, micro hardness, microstructure, etc. The results showed that the changing trend of the cutting forces in three directions is HY-103 > Vasco 7000 > TRIM E709. The TRIM E709 contains the maximum lubricants, which reduce cutting force and Sa roughness, while the Vasco 7000 contains the minimum corrosive elements, which results in the least pitting. Both tangential and axial stresses under cutting fluid are tensile stresses. TRIM E709 and Vasco 7000 are reduced axially by 4.45% and 7.60% relative to HY-103, respectively. The grain refinement layer depths of HY-103, TRIM E709, and Vasco 7000 are 9 μm, 4 μm, and 8 μm, respectively, and TRIM E709 can induce recrystallized grains to grow along {001} of the sample cross section, which results from the lowest cooling rate. This work may provide an innovative control strategy for cutting fluid to improve surface integrity and service performance.

This paper investigates the reduction of springback by an electrically single-pulsed current for an ultra-high-strength martensitic steel sheet, MART1470 1.2t. In order to evaluate the springback reduction by the electric current, V-bending tests were performed with various parameter-sets (current density and pulse duration). The amount of springback reduction was then calculated from the measured bent-angle of tested specimens. Experimental results show the springback is reduced with the increase in the current density, the pulse duration, and the electric energy density. In order to clarify thermal and athermal portions in the effect of electric current on the springback reduction, two ratios of force and isothermal flow stress were calculated based on bending theory. From the comparison of the ratios, it is noted that the athermal portion mainly contributes to the force relaxation, so the springback amount decreases. The athermal portion significantly increases as the electric energy density increases. Microstructures and micro-Vickers hardness were observed to confirm the applicability of the single-pulsed current to forming processes in practice. The springback reduction can be achieved up to 37.5% without severe changes in material properties when the electric energy density increases up to 281.3 mJ/mm3. Achievable reduction is 85.4% for the electric energy density of 500 mJ/mm3, but properties remarkably change. [ABSTRACT FROM AUTHOR]

This study investigates the microstructure and the hardness of thermomechanically processed joints of ultra-high-strength steel (UHSS) S1100MC, which were butt-welded by means of metal active gas (MAG) welding and electron beam welding (EBW). In MAG welding, the microstructure of the fusion zone (FZ) consisted predominantly of fine ferrite grains. Due to the formation of martensite/austenite (M/A) constituents, the hardness of the reheated weld metal (WM) was higher for vertical-up (PF) than for horizontal flat (PA) welding position. The microstructure of the heat-affected zone (HAZ) of the last welding pass consisted mainly of hard martensite in consequence of fast cooling. However, necklace-type M/A constituents and bainite phases of lower hardness formed when the HAZ of prior welding passes were reheated by subsequent welding passes. The microstructure of the EBW fusion zone was dominated by martensite. Therefore, the fusion zone had considerable higher hardness than the base material (BM). The width of the HAZ in EBW weldments is less than in MAG weldments. [ABSTRACT FROM AUTHOR]