Ultra-high Strength Steel Research Articles

Development of steel for hot stamping of 22mnb5 steel by joule effect heating

With the increase in demand and development of the automobile sector, where it becomes important to improve the efficiency of energy consumption of vehicles, one can observe the need for materials that are both light and have high mechanical strengths. The Ultra High Strength Steel (UHSS), which include TWIP (Twinning-Induced-Plasticity) and Hot Stamping are characterized by unique microstructures and metallurgical properties that allow car manufacturers to meet the diverse functional requirements of vehicles. Boron steel is suitable for mechanical hot forming. In the last few years, hot stamped parts have occupied a prominent place in the bodies of bodies due to being in line with the demands mentioned above. There is a lot of research under development for this technology, be it in materials, means of production, coatings and applications. Currently, there is a patent that generates a technological restriction for the processing of hot stamping; in which it is to use boron steel with Al-Si metallic coating and heating the blank to be hot stamped through a radiation furnace. The objective of this work is to use a new processing alternative through another metallic diffusion coating of Fe-Zn and heating of the blank to be hot stamped by Joule effect; generating an industrial experiment and prototyping for characterization. The results showed the feasibility of using boron steel with Fe-Zn diffusion coating in hot forming applications by heating by Joule effect, with good surface quality, meets the mechanical properties and microstructure, without cracks in the steel, excellent adhesion of the paint layer and good resistance to corrosion.

Influence of isothermal tempering on microstructures and hydrogen-environmentally embrittlement susceptibility of laser additively manufactured ultra-high strength AerMet100 steel

Good hydrogen-environmentally embrittlement (HEE) resistance is necessary for safe service of critical main load-bearing ultra-high strength steel (UHSS) components of advanced aircrafts in marine atmosphere environment. Controlling isothermal tempering time is one important heat treatment method to optimize HEE resistance of UHSS by designing microstructure. Influence of isothermal tempering time at 482 °C on microstructures and HEE susceptibilities of laser additively manufactured (LAM) AerMet100 steel was investigated by several material characterization methods and slow strain rate tensile (SSRT) tests in both of air and 3.5 wt% NaCl aqueous solution. Results mainly indicate that microstructure evolution behaviors of LAM AerMet100 steel successively include martensite and retained austenite decomposition, M2C carbide formation and coarsening, and film-like reverted austenite formation and thickening. Short-time tempering specimens of LAM AerMet100 steel has the highest strength-loss-index in all of the specimens due to the lowest austenite amount. The lower HEE susceptibility of LAM AerMet100 steel in long-time tempering condition is mainly corresponding to M2C carbide coarsening and film-like reverted austenite thickening. Furthermore, difference of hydrogen charging method strongly influences macro-fracture behaviors of the SSRT specimens, but micro-cracking modes of the steel in different conditions are predominantly martensite packet/plate boundary cracking. Under open-circuit potential condition, HEE cracking zone is prone to be origin from pitting corrosion sites, and the SSRT specimen in long-time tempering condition have a good ductility; in contrast, under cathodic polarization condition, HEE cracking zone is around the circumferential zones, and the tensile specimens are brittle fracture. These findings are important when considering achieving improved HEE resistance for LAM AerMet100 steel tempered at 482 °C of a longer isothermal tempering time at the strength level of interest.

Experimental study on surface integrity and fatigue life of an ultra-high strength steel by the composite strengthening process of pre-torsion and ultrasonic rolling

A new surface modification method, i.e., the composite strengthening process of pre-torsion and ultrasonic rolling, is proposed for surface strengthening of an ultra-high strength steel (45CrNiMoVA). Based on the enhancement of dislocation density in the internal structure of the material by pre-torsion strengthening process and the grain refinement effect of ultrasonic rolling strengthening process, the evolution law of surface integrity and fatigue life of ultra-high strength steel after the composite strengthening process of pre-torsion and ultrasonic rolling were analyzed. The results show that the composite strengthening process has an obvious surface finishing effect, in which ultrasonic rolling plays a key role. The coupling effect of pre-torsion and ultrasonic rolling causes the residual compressive stress in the surface layer of ultra-high strength steel with large amplitude and deep influenced layer. Compared with the ultrasonic rolling process, composite strengthening can produce deeper plastic deformation on the surface of ultra-high strength steel. After composite strengthening, the high cycle fatigue life of the specimens increases greatly, and the torsional fatigue fracture mode changes to quasi-cleavage fracture. In conclusion, the composite strengthening process of pre-torsion and ultrasonic rolling is of great significance to improve the surface integrity and fatigue life of the ultra-high strength steel.

Study on mode I dynamic fracture characteristics with a mini three-point bending specimen for the split Hopkinson bar technique

In recent years, dynamic fracture experimental techniques have been developed rapidly in the field of engineering and scientific research. However, there still exist some difficulties in dynamic fracture tests, such as the limitation of loading rate, the accuracy of transmitted wave, the difficulty of crack initiation, and etc. To solve such problems, a novel mode I dynamic fracture experimental technique is proposed in this work, with a mini three-point bending(3PB) specimen and two kinds of fixtures specially designed for the split Hopkinson pressure bar (SHPB) system. In this method, the dynamic stress intensity factor curve (DSIF) is obtained by the experimental-numerical method, and the crack initiation time is determined by the strain gauge method. To verify the feasibility and reliability of the proposed method, the dynamic fracture toughness (DFT) of two kinds of ultra-high-strength steel (UHSS) is determined by this new method and then compared with the results of other methods. Meanwhile, the loading rate effect of fracture toughness, the size effect of crack initiation time and the failure mechanisms of the two materials under mode I fracture are also studied. The results show that fracture toughness depends on the energy consumed to form two types of fracture regions within the fracture initiation time. This work provides a new method for studies of the dynamic fracture behaviors of brittle solids.

Atomic-scale understanding of solute interaction effects on grain boundary segregation, precipitation, and fracture of ultrahigh-strength maraging steels

Understanding the fundamental mechanisms of embrittlement and de-embrittlement is crucial for the development of strategies toward advanced steels with improved performance. In this study, the solute interaction effects on grain boundary (GB) segregation, precipitation, and fracture of Fe–Ni–Ti–(Mo) maraging steels were systematically investigated through a combination of experimental and theoretical techniques, including scanning transmission electron microscopy, atom probe tomography, and first-principles calculations. Our results reveal that the Fe–Ni–Ti maraging steel suffers from serious intergranular embrittlement and the mechanism is related to the formation of coarse Ni3Ti precipitates and associated precipitate-free zones (PFZs) at GBs, which facilitate the crack initiation and growth along the GBs. Interestingly, adding Mo to the maraging steel effectively suppresses the intergranular embrittlement, thereby substantially improving the ductility. Atomistic analyses reveal that Mo de-embrittles the GBs by reducing the segregation of Ni and Ti, which substantially inhibits the formation of coarse Ni3Ti precipitates and PFZs at the GBs, thereby alleviating their harmful impact on the GB cracking. In addition, the Mo segregation enhances the GB cohesion, which may play a minor role in suppressing the GB fracture.

Research on Deep Hole Machining Based on Aircraft Landing Gear

A landing gear is the main large bearing structural part of an aircraft. The change of material and structure makes the processing of the aircraft landing gear more difficult. Aiming at the structural characteristics of the aircraft landing gear deep cavity, the deep hole processing technique of a 300M steel workpiece was studied. A separated NC deep hole drilling and boring machine was adopted to process the workpiece. Cavity hole special cutting tools were introduced. Fixture and sealing mode were improved. NC programs were optimized. Thus, the processing problems of deflection, vibration, chip breaking and chip removal were solved. The workpiece with width to diameter ratio greater than 10 and surface roughness of hole is 3.2 μm was successfully machined by machining experiment, and the high efficiency machining of deep hole of ultra-high strength steel was realized. Thus the theoretical foundation is laid for the machining of complex cavity deep holes for cutting high-strength alloy materials.

Effects of Edge Heating and Strain Gradient on Stretch Flange Deformation Limit of Steel Sheet

To improve the stretch flange deformation limit of ultrahigh-strength steels, a partial edge heating method was investigated. The effect of the strain gradient on the stretch flange deformation limit when a partial edge heating method was applied was investigated by the hole expansion test and FEM analysis. The stretch flange formability was improved by application of a partial edge heating method because of the recovery of the shearing microstructure and the reduction of the hardness difference of microstructure. It was found that the improvement of the deformation limit strain for the strain gradient with edge heating depends on the strain gradient region. In the low strain gradient region, the improvement margin of the deformation limit strain with edge heating was small. On the other hand, in the high strain gradient region, the deformation limit strain was markedly improved compared with that without heating. It was confirmed that the strain gradient in the direction perpendicular to the maximum principal strain could be used for the prediction of the deformation limit and also the determination of the stretch flange fracture by the FEM analysis of actual parts even under edge heating conditions.

Effects of Deep Cryogenic Treatment and Controlled Rolling on Microstructures and Mechanical Properties of an Ultra-High Strength Steel

The microstructure mechanisms and mechanical properties of 0.23C-1.96Si-1.94Cr-1.93 Mn-0.35 Mo ultra-high strength steel treated by the deep cryogenic treatment at −196 °C were investigated after the steel was hot rolled at different temperatures. Experimental results show that austenitizing zone rolling with a large reduction in a single pass can comprehensively enhance the mechanical properties due to the high volume of retained austenite and refined lath martensite and bainite. The high strain gradient was suppressed, and tensile strength, yield strength, impact toughness, and total elongation were 2221 MPa, 2017 MPa, 65.5 J, and 16.9%, respectively. In addition, the austenitizing zone rolling can promote the formation of film retained austenite more than dual phase zone rolling, and retained austenite was decreased with an increase in rolling pass in a total rolling reduction of 75%. It is demonstrated that deep cryogenic treatment after austenite zone rolling with a large reduction is the proper method to enhance strength and toughness via refinement strengthening and retained austenite. This work not only reveals that the effects of refinement strengthening and high strain gradients can be markedly improved by deep cryogenic treatment after austenitizing zone rolling with large reduction strategies, but also provides a realistic preparation technology for the exploitation of superior steel applications.

Evaluation of Riveting Force Influence on the Quality of Riveted Joint of Aluminium Alloy EN AW - 6016

Joining technologies are very important aspects of production process in the automotive. Especially regarding the use of newly developed types of materials (e.g. ultra high-strength steels or aluminium alloys), in addition to welding technology, e.g. riveting or adhesive bonding technologies are used. Submitted article evaluates the effect of the riveting force on the final quality of riveted joint when joining aluminium alloy EN AW-6016 (thickness 2 mm). The actual evaluation of the riveted joint was carried out using a shear test, measuring the hardness HV01 and deformation analysis of specimen using non-contact optical scanning (ATOS III Triple Scan system).

Microstructure-mechanical properties relationships and strain hardening mechanism for medium-Mn steel with low yield and ultra-high tensile strength

Considering the problems of forming ultra-high strength steel, a medium-Mn steel with low yield strength was prepared. The steel exhibited a yield strength of 537 MPa, a tensile strength of 1780 MPa, and an elongation of 17.8%. The study aimed to identify the key factors contributing to the low yield-to-tensile ratio and strain hardening behavior of the steel. To achieve this, interrupted tensile tests, loading-unloading-reloading tensile tests, and microstructure characterization were used, along with theoretical calculations. The results indicated that the low dislocation density in lath ferrite, initial martensite, and retained austenite, as well as the high-volume fraction of retained austenite, were the primary factors contributing to the low yield strength. During tensile deformation, strong solid solution strengthening in strain-induced martensite was the main factor driving strain hardening. Additionally, the rapid multiplication of dislocations in ferrite and martensite, along with the heterogeneous deformation-induced hardening caused by the heterogeneous structure composed of strain-induced martensite and other phases, were also significant factors contributing to strain hardening. When considering grain boundary strengthening, dislocation strengthening, solid solution strengthening, and heterogeneous deformation-induced hardening, the theoretical yield strength and flow stress calculated in this study closely matched the actual measured values.

Investigation of the effects of lubrication type, blankholder force and force holding time on the spring-back behaviour for hat-shaped MART steels

ABSTRACT In this study, hat-shaped bending experiments have been conducted on MART1200 and MART1400 Ultra-High-Strength Steels (UHSS). To investigate the influence of different lubrication conditions, on the spring-back, three different lubricant types (Graphite, Teflon film and Teflon film + Graphite) have been used in the experiments. A constant 6.5 tons of bottoming force has been applied at the end of the bending stroke and the force has been held on the samples for 0, 10s and 60s to observe the effect of force holding time on the spring-back. In addition, the effect of blankholder force on the spring-back has also been investigated for MART1400 steel. It has been found that the decrease of blankholder force and the friction between the die and sheet interfaces has considerably reduced the spring-back in the sidewalls and flange regions and also lowered the side wall curl. It has been revealed through finite element analysis that the sheet bottom has suffered a larger deformation in lower friction conditions and blankholder forces and caused in the reduction of spring-back.

Enhancement of shape accuracy and die quenchability of ultra-high strength steel hollow products in hot stamping of tubes using eco-friendly fiber-reinforced ice mandrel

A hot stamping process of quenchable steel tubes using a mandrel reinforced with eco-friendly fibers was developed to produce ultra-high strength steel hollow parts having enhanced lightweighting and crashworthiness. High internal pressure was generated to improve the die quenchability and shape accuracy of the formed parts by the fiber reinforcement. Wood sawdust, shredded copy paper, and plant fiber made of recycled toilet paper were chosen as the fibers, and not only the strength was evaluated from a uniaxial compression test but also the melting behavior of the mandrel was examined. The influence of the fiber reinforcement on the shape accuracy and die quenchability of hot-stamped parts was investigated. The generated internal pressure with the fiber-reinforced ice mandrel was higher than that with the pure ice mandrel without the reinforcement, and thus, the shape accuracy and die quenchability of hot-stamped parts were significantly improved even for a comparatively small change in internal volume of tubes.

Bond Behavior Between Recycled Concrete and Corroded Ultrahigh-Strength Steel Bars

Bond Behavior Between Recycled Concrete and Corroded Ultrahigh-Strength Steel Bars

Critical effect of heat input on joint quality in resistance element welding of Al and steel

Resistance element welding (REW) has recently attracted increasing interest, especially in the joining of high-strength steels and aluminum alloys. However, the intensity of heat input is a critical factor for the joint quality due to the possibility of excessive melting of the aluminum sheet around the element shank, which often degrades the joint's performance. This study evaluates the influence of heat input on the joint formation and mechanical performance of REW joints between ultra-high strength steel and aluminum alloy. In addition to the experimental investigations, a finite element method (FEM) was used to analyze the temperature distribution, nugget formation, and aluminum sheet melting characteristics as a function of heat input. The results show that the nugget dominates the joint's strength if the heat input is comparatively low. When heat is increased, however, the aluminum sheet plays a significant role in the joint's strength. Excessive melting of aluminum sheets reduces the load-bearing area of the joint. The competition between the weld nugget and aluminum sheet determines the joint's strength. The failure mode changes from interfacial to aluminum sheet failure as the heat input increases. The combined effect of aluminum sheet thickness and heat input on the joint strength was also evaluated. The results indicate that determining the appropriate heat input level is crucial to achieving an excellent performance of the REW joints.

Study on Corrosion Behavior and Mechanism of Ultrahigh-Strength Hot-Stamping Steel Based on Traditional and Compact Strip-Production Processes.

Hot-stamping steel is a type of high-strength steel that is mainly used in key safety components such as the front and rear bumpers, A-pillars, and B-pillars of vehicles. There are two methods of producing hot-stamping steel, i.e., the traditional process and the near net shape of compact strip production (CSP) process. To assess the potential risks of producing hot-stamping steel using CSP, the microstructure and mechanical properties, and especially the corrosion behavior were focused on between the traditional and CSP processes. The original microstructure of hot-stamping steel produced by the traditional process and the CSP process is different. After quenching, the microstructures transform into full martensite, and their mechanical properties meet the 1500 MPa grade. Corrosion tests showed that the faster the quenching speeds, the smaller the corrosion rate of the steel. The corrosion current density changes from 15 to 8.6 μA·cm-2. The corrosion resistance of hot-stamping steel produced by the CSP process is slightly better than that of traditional processes, mainly since the inclusion size and distribution density of CSP-produced steel were both smaller than those of the traditional process. The reduction of inclusions reduces the number of corrosion sites and improves the corrosion resistance of steel.

Elastic wave properties in ultra-high strength steel (HV670) exposed to various corrosive solutions

Elastic wave properties in ultra-high strength steel (HV670) exposed to various corrosive solutions

Comparative Study of Wire Arc Additive Manufactured and Wrought Ultrahigh-Strength Steels: Flow Behavior and Microstructure Evolution

Comparative Study of Wire Arc Additive Manufactured and Wrought Ultrahigh-Strength Steels: Flow Behavior and Microstructure Evolution

Experimental Study of the Postfire Mechanical Properties of Grade 14.9 Superhigh-Tension Bolt

Fire hazard has been one of the most critical factors to threaten the safety of steel structures. Steel structures may not collapse when suffering fire and cooling down, but the serviceability of building after the fire needs to be evaluated before reusing. Hence, the postfire mechanical properties of steel are necessary. With the high- and ultrahigh-strength steel applied in the construction industry, superhigh-tension bolts (SHTB) are thus encouraged to apply in engineering. However, the current studies on the postfire mechanical properties mainly focus on the Grade 8.8s and 10.9s high-strength bolts. There is limited research work conducted to study the postfire mechanical properties of SHTB. In this study, coupon tests of Grade 14.9 SHTB were conducted, and the postfire performance was evaluated. Failure mode, stress-strain curve, elastic modulus, yield stress, tensile strength, tensile strain, percentage elongation after fracture, and percentage reduction of area were tested and analyzed. The postfire mechanical properties of different types of bolts were compared. The test results indicated that temperatures beyond 750°C cause decrease in the ductility of Grade 14.9 SHTB, regardless of the cooling condition. Accordingly, predictive formulas with good precision are proposed for the postfire mechanical properties of Grade 14.9 SHTB cooled down by air and water, respectively.

Development of a reversible top-hat forming approach for reducing flange wrinkling in flexible roll forming

Flexible roll forming (FRF) addresses the need for flexible manufacturing and light-weighting in electric vehicle production, but its widespread implementation is prevented by severe wrinkling in the flange. Wrinkling in sheet metal forming depends on the tool support and material deformation. In FRF the tool effects on wrinkling are complex, given that the sheet undergoes unsupported deformation when it enters and exits the forming roll and there is no fundamental understanding of material deformation in the critical forming regions where wrinkling occurs. This prevents an effective tool and process design. This work develops a new model to analyse the stability of the flange by combining plastic bifurcation analysis with thin-shell assumptions. The model defines a stress ratio parameter that can be used to determine when in the process, deformation becomes unstable considering the tool support conditions and material deformation. Based on this a new approach for wrinkling reduction is developed that includes the forming and bending back of a top-hat shape to provide additional flange stiffness when wrinkling is likely to initiate. The top-hat shape is optimised using a series of linear perturbation ‘buckle’ analyses to provide the highest possible stiffness increase in the flange. This is followed by experimental manufacturing trials to test and prove the new concept for wrinkling reduction and to validate the model. It is shown that the new top-hat forming approach can eliminate wrinkling in the flange even when flexible roll forming Ultra High Strength Steels. The analytical model results suggest that this is due to the stiffening effect of the top-hat and the reduction of the effective flange length which decreases the stress ratio. The new model in combination with the new forming approach to eliminate flange wrinkling represents a vital step towards a more effective process and tool design that will enable the widespread application of FRF in automotive manufacturing.

Enhanced mechanical properties of a high-carbon martensite steel processed by heavy warm rolling and tempering

The strength-ductility trade-off in steel, especially high-carbon steel, has been a long-standing and troubling issue; steel with increased mechanical properties has attracted much attention in recent years. In this study, an ultrafine-grained ultrastrong high-carbon steel with good ductility was produced by stabilizing retained austenite (RA) and accelerating carbide precipitation by warm rolling, quenching and tempering. The ultimate tensile strength reaches approximately 2550 MPa, with a total elongation of 5.1% after tempering at 220 °C. The corresponding microstructure was composed of stable high-carbon RA and ultrafine-grained tempered martensite with carbide precipitates. The thermodynamic stability of RA was increased by carbon partitioning during quenching. The volume fraction was optimized to 6%. As the grain size of the primary austenite was refined to approximately 3 μm or smaller by warm rolling along the normal direction, the substructure of the high-carbon martensite was characterized as ultrafine twins and high-density dislocations. Carbide precipitations from the martensite matrix are accelerated at lower tempering temperatures. The main strengthening mechanism was still the strengthening of the solid solution and dislocations. Good tensile ductility was mainly derived from the effective weakening of the solid solution of martensite, whose carbon content decreased from 0.86 wt% to 0.61 wt%. Additionally, the greater ductility was attributed to the transformation-induced plasticity effect of larger amounts of high-carbon RA. These results may help in the design and production of ultrahigh strength and ductile high-carbon steel.