Advanced Ultra-high Strength Steels Research Articles

Effect of microstructure evolution on impact toughness in advanced ultra-high strength steel

The microstructure evolution and strength-toughness matching relationship of advanced ultra-high strength steel were studied under the tempering temperature range of 200–400 °C. The Charpy impact absorption energy of U-notch and V-notch focused on the crack initiation and crack propagation processes, respectively. The peak impact absorption energy of the U-notch appeared at 300 °C due to reduced stress concentration from low GND (Geometrically Necessary Dislocations) density. The optimal mechanical properties were found after tempering at 250 °C, where martensitic blocks hindered crack propagation, resulting in the peak impact energy of V-notch.

Exploring the effect of alloying elements on the thermoelasticity and strength of bcc Fe-based alloys by first-principles phonon calculations

Designing compositions for novel alloys requires a fundamental grasp of the role of each alloying element in its properties. This study delves into the influence of alloying elements in Ultra-High Strength Steels on essential high-temperature properties, including thermoelasticity, thermal stress, and strength, utilizing first-principles phonon calculations. These properties are pivotal for steels used in high-temperature applications. Our investigation quantitatively forecasts the impact of alloying elements on elastic properties, providing valuable insights for advanced alloy design in high-temperature environments. The introduction of alloying elements markedly amplifies the elastic modulus of Fe-based alloys. The Ni, Mo, Cr, and C elements significantly enhance the anisotropy of the alloy. While refining elastic modulus and mechanical properties, alloying elements induce thermal stress, posing a potential risk of high-temperature cracking. The quantitative analysis underscores the robust strengthening potential for C, Ni, Mo, and W elements, with interstitial C emerging as pivotal in enhancing mechanical properties. The findings of this study illuminate the diverse roles played by different alloying elements and offer theoretical guidance for the design of advanced Ultra-High Strength Steels.

Interfacial Characterization and Mechanical Properties of Friction Stir Welding Dissimilar Joint Between Al Alloy and Advanced Ultrahigh-Strength Steel

Interfacial Characterization and Mechanical Properties of Friction Stir Welding Dissimilar Joint Between Al Alloy and Advanced Ultrahigh-Strength Steel

A novel low-cost ultra-strong maraging steel by additive manufacturing

A novel Co/Mo free maraging steel, hardened by nano-sized β-NiAl precipitates, has been developed for additive manufacturing (AM). The composition was designed to achieve ultra-high strength with low cost, simultaneously maintaining the formability. The steel was successfully fabricated using laser powder bed fuse (L-PBF) additive manufacturing (AM) technology, and the effect of heat-treatment on the microstructure and mechanical properties of fabricated samples was investigated. Due to the refined microstructure and high-density dislocations, the as-built samples displayed in excess of 1400 MPa ultimate tensile strength with about 16% total elongation. This is the highest reported strength to date for any as-built AM materials. Upon direct aging treatment, nanoscale ordered β-NiAl precipitated in the martensite matrix, resulting in a remarkable combination of mechanical properties, i.e. the strength levels exceeding 2000 MPa and elongation up to ∼8% at room temperature. This work contributes to the development of advanced ultrahigh-strength steel for AM.

Equivalent Energy Absorption—A Methodology for Improved Automotive Crash Safety Design

Abstract Safety norms across the world are becoming more and more stringent posing new challenges to achieve lightweight vehicle structures. Structures made of advanced/ultra-high strength steels (AHSS) play a vital role in meeting the vehicle safety targets, by absorbing large amounts of impact energy, as well as by withstanding higher impact loads that occur due to vehicle collisions. Safety simulations usually take longer solution times due to their complexity and nonlinear nature. Engineers often encounter with a problem of quick evaluation of safety performance using different grades of materials to optimize the weight and cost. A new methodology—equivalent energy absorption (EEA)—has been proposed to do a quick trade-off study on performance versus weight for various thicknesses and material combinations. A relationship is established between the gauge and grade of a component to derive an equivalent safety performance so that engineers can make quick decisions by conducting minimal number of simulations. A simple rectangular crush box was considered for study to assess the energy absorption (EA) with various material and thickness combinations. A design of experiments (DOEs) study was done using simulations with many numbers of material grades and gauges to construct a 3D response surface between gauge grade and EA parameters to understand the relationship between each of these parameters. A case study has been discussed in the paper about application of this methodology on a vehicle to evaluate its safety performance. It was found that more than 80% evaluation time is reduced using this methodology.

Exploring New Strategies for Ultrahigh Strength Steel via Tailoring the Precipitates

Ultrahigh-strength (UHS) steels have shown great potential in the field of high-end equipment manufacturing in demand of lightweight engineering and performance upgrade. A significant research effort has been directed toward the development of advanced UHS steels with excellent combination of strength and toughness. In the course of development, tailoring precipitates by means of composition design and process optimization is absolutely a critical module. In this mini review, typical UHS steels strengthened by carbides and intermetallics are systematically summarized and discussed. With the increase of strength, the toughness losses of UHS steels strengthened by carbides and intermetallics have been compared in detail. In particular, the in-depth mechanisms leading to various strength/toughness variation trends have been discussed, extracting the bottleneck in developing new-generation UHS steels containing merely one type of precipitate. Meanwhile, prospects on designing advanced UHS steels strengthened by coexisting dispersive precipitates have been proposed to achieve better performance.

Unveiling Interactions of Non-Metallic Inclusions within Advanced Ultra-High Strength Steel: A Spectro-Microscopic Determination and First-Principles Elucidation

Determining non-metallic inclusions (NMIs) are essential to engineer ultra-high-strength steel as they play decisive role on performance and critical to probe via conventional techniques. Herein, advanced Synchrotron X-ray absorption coupled with photoemission electron microscopy and first-principles calculations are employed to provide the structure, local bonding structure and electronic properties of several NMI model systems and their interaction mechanism within and the steel matrix. B K-, N K-, Ca L2,3- and Ti L2,3-edge spectra show that the additional B prefers to result in h-BN exhibiting strong interaction with Ca2+. Such Ca2+-based phases also stabilize though TiN, revealing the irregular coordination of Ca2+. Observed intriguing no interaction between TiN and h-BN is further supported with the first-principles calculations, wherein unfavorable combination of TiN and h-BN and stabilization of bigger sized Ca2+-based inclusions have been found. These observations can help to optimize the interaction mechanism among various inclusions as well as steel matrix.

Microstructural Evolution and Mechanical Behavior of Friction-Stir-Welded DP1180 Advanced Ultrahigh Strength Steel

Friction stir lap welding of a DP1180 advanced ultrahigh strength steel was successfully carried out by using three welding tools with different pin lengths. The effects of the welding heat input and material flow on the microstructure evolution of the joints were analyzed in detail. The relationship between pin length and mechanical properties of lap joints was studied. The results showed that the peak temperatures of all joints exceeded Ac3, and martensite phases with similar morphologies were formed in the stir zones. These martensite retained good toughness due to the self-tempering effect. The formation of ferrite and tempered martensite was the main reason for the hardness reduction in heat-affected zone. The mechanical properties of the lap joints were determined by loading mode, features of lap interface and the joint defects. When the stir pin was inserted into the lower sheet with a depth of 0.4 mm, the lap joint exhibited the maximum tensile strength of 12.4 kN.

Experimental and Numerical Investigations of Applying Tip-bottomed Tool for Bending Advanced Ultra-high Strength Steel Sheet

This research was carried out aiming to investigate the application of a tip-bottomed tool for bending an advanced ultra-high strength steel sheet. The V-die bending experiment of a dual phase steel (DP980) sheet which had a thickness of 1.6 mm was executed using a conventional bending and a tip-bottomed punches. Experimental results revealed that the springback of the bent worksheet in the case of the tip-bottomed punch was less than that of the conventional punch case. To further discuss bending characteristics, a finite element (FE) model was developed and used to simulate the bending of the worksheet. From the FE analysis, it was found that the application of the tip-bottomed punch contributed the plastic deformation to occur at the bending region. Consequently, the springback of the worksheet reduced. In addition, the width of the punch tip was found to affect the deformation at the bending region and determined the springback of the bent worksheet. Moreover, the use of the tip-bottomed punch resulted in the apparent increase of the surface hardness of the bent worksheet, compared to the bending with the conventional punch.

Co-precipitation of nanoscale particles in steels with ultra-high strength for a new era

Advanced ultra-high strength steels are highly desirable for a wide range of engineering applications. Nanoscale co-precipitation strengthening in steels has received increasing attention in recent years and has become a new cornerstone for the development of advanced steels with superior combination of mechanical, welding, and irradiation properties for a new era. In this review, we highlight recent advances in computation-aided alloy design, nanostructural characterization, and unique properties of newly developed nanoscale co-precipitation-strengthened steels. In particular, our emphasis is on elucidating alloy design strategies, the co-precipitation mechanism, and cooperative evolution of multiple types of nanoparticles, and the correlation between nanostructures and bulk steel properties. Finally, future research areas for this class of nanostructured steels are critically discussed.

Boron Effect on the Softening Parameter (Ω) of Advanced Ultra-High Strength Steels (A-UHSS) under Uniaxial Hot-Compression Conditions

ABSTRACTAdvanced ultra-high strength steels (A-UHSS) are revolutionizing both the steel and automotive industries, therefore it is imperative to study their hot plastic deformation behavior and modeling. The flow characteristics of all hot forming processes consist basically of two competitive phenomena: strain hardening and softening due to dynamic mechanisms (recovery and/or recrystallization). In this research work, the softening parameter was determined in a low carbon A-UHSS microalloyed steel with different amounts of boron (0, 14 and 214 ppm). Experimental stress–strain data of uniaxial hot-compression tests at different temperatures (950, 1000, 1050 and 1100 °C) and strain rates (10–3, 10–2 and 10–1 s–1) were used. The stress–strain relationships as a function of temperature and strain rate were described on the basis of the Estrin, Mecking, and Bergström model. The experimental values of the softening parameter Ω were adjusted using the least-squares method. In general, the results reveal that the softening parameter increases with increasing boron content.

Influence of Boron on the Precipitation Kinetics in Advanced Ultra-High Strength Steels

ABSTRACTIn the present work, the stress relaxation method was employed to determine the influence of B addition on the kinetics of strain-induced precipitation and its interaction with the static austenite recrystallization. For this purpose, the behavior of two low carbon advanced ultra-high strength steels was analyzed during stress relaxation tests at different temperatures and constant pre-strain rate. The precipitation start (Ps) and finish (Pf) times were determined from the relaxation curves and then the corresponding precipitation-time-temperature diagrams were constructed for each steel. Transmission Electron Microscopy was used to determine the chemical nature and evolution of precipitation. In general, the results show that the addition of B retards the austenite recrystallization, tends to accelerate the precipitation kinetics of carbonitrides and leads to a finer and denser distribution of precipitates. These results are discussed in terms of the driving force for the nucleation of precipitation, which in turn is controlled by the degree of supersaturation of microalloying element and as a function of B segregation and B-vacancy complexes to dislocations and grain boundaries.

Modeling of the hot flow behavior of advanced ultra-high strength steels (A-UHSS) microalloyed with boron

In this research work, modeling of the hot flow behavior was carried out in a low carbon advanced ultra-high strength steels (A-UHSS) microalloyed with different amounts of boron (14, 33, 82, 126 and 214ppm). For this purpose, experimental stress–strain data of uniaxial hot-compression tests over a wide range of temperatures (1223, 1273, 1323 and 1373K (950, 1000, 1050 and 1100°C)) and strain rates (10−3, 10−2 and 10−1s−1) were used. The stress–strain relationships as a function of temperature and strain rate were successfully described on the basis of the approach proposed by Estrin, Mecking, and Bergström, together with the classical Avrami equation and the conventional hyperbolic sine function. The analysis of the modeling parameters of the hot flow curves shows that boron additions to A-UHSS play a major role in softening mechanisms rather than on hardening. The peak stress (σp) and steady-state stress (σss) values show a decreasing trend with increasing boron content, which indicates that boron additions promote a solid solution softening effect additional to that produced by DRX. The time for 50% recrystallization (t50%) tends to increase with boron additions, revealing that boron additions cause a delay of the DRX kinetics during hot deformation. Similarly, the presence of boron in the steel decreases the apparent activation energy for recrystallization (Qt), indicating that boron additions accelerate the onset of DRX. The constitutive equations developed in this way provided an excellent description of the experimental hot flow curves.

Effect of Boron on the Hot Ductility Behavior of a Low Carbon Advanced Ultra-High Strength Steel (A-UHSS)

This research work studied the effect of boron additions (14, 33, 82, 126, and 214 ppm) on the hot ductility behavior of a low carbon advanced ultra-high strength steel. For this purpose, specimens were subjected to a hot tensile test at different temperatures [923 K, 973 K, 1023 K, 1073 K, 1173 K, and 1273 K (650 °C, 700 °C, 750 °C, 800 °C, 900 °C, and 1000 °C)] under a constant true strain rate of 10−3 s−1. The reduction of area (RA) of the tested samples until fracture was taken as a measure of the hot ductility. In general, results revealed a marked improvement in hot ductility from 82 ppm B when the stoichiometric composition for BN (0.8:1) was exceeded. By comparing the ductility curve of the steel with the highest boron content (B5, 214 ppm B) and the curve for the steel without boron (B0), the increase of hot ductility in terms of RA is over 100 pct. In contrast, the typical recovery of hot ductility at temperatures below the Ar3, where large amounts of normal transformation ferrite usually form in the structure, was not observed in these steels. On the other hand, the fracture surfaces indicated that the fracture mode tends to be more ductile as the boron content increases. It was shown that precipitates and/or inclusions coupled with voids play a meaningful role on the crack nucleation mechanism, which in turn causes hot ductility loss. In general, results are discussed in terms of boron segregation and precipitation on austenitic grain boundaries during cooling from the austenitic range and subsequent plastic deformation.

Dynamically recrystallized austenitic grain in a low carbon advanced ultra-high strength steel (A-UHSS) microalloyed with boron under hot deformation conditions

ABSTRACTThis research work studies the dynamically recrystallized austenitic grain size (Drec) in a new family of low carbon NiCrCuV advanced ultra-high strength steel (A-UHSS) microalloyed with boron under hot deformation conditions. For this purpose, uniaxial hot-compression tests are carried out in a low carbon A-UHSS microalloyed with different amounts of boron (14, 33, 82, 126 and 214 ppm) over a wide range of temperatures (950, 1000, 1050 and 1100°C) and constant true strain rates (10−3, 10−2and 10−1s−1). Deformed samples are prepared and chemically etched with a saturated aqueous picric acid solution at 80°C in order to reveal theDrecand examined by light optical (LOM) and scanning electron microscopy (SEM). TheDrecis related to the Zener-Hollomon parameter (Z), and thereafter theDrecdivided by Burger's vector (b) is related to the steady state stress (σss) divided by the shear modulus (µ) (Derby model). Results shown that theDrecin the current steels is fine (≈ 23 μm) and almost equiaxed, and the recrystallized grain size-flow stress relationship observed after of plastic deformation is consistent with the general formulation proposed by Derby. It is corroborated that boron additions to the current A-UHSS do not have meaningful influence on theDrec.

Effect of boron on the continuous cooling transformation kinetics in a low carbon advanced ultra-high strength steel (A-UHSS)

ABSTRACTThe aim of the present research work is to investigate the influence of B addition on the phase transformation kinetics under continuous cooling conditions. In order to perform this study, the behavior of two low carbon advanced ultra-high strength steels (A-UHSS) is analyzed during dilatometry tests over the cooling rate range of 0.1-200°C/s. The start and finish points of the austenite transformation are identified from the dilatation curves and then the continuous cooling transformation (CCT) diagrams are constructed. These diagrams are verified by microstructural characterization and Vickers micro-hardness. In general, results revealed that for slower cooling rates (0.1-0.5 °C/s) the present phases are mainly ferritic-pearlitic (F+P) structures. By contrast, a mixture of bainitic-martensitic structures predominates at higher cooling rates (50-200°C/s). On the other hand, CCT diagrams show that B addition delays the decomposition kinetics of austenite to ferrite, thereby promoting the formation of bainitic-martensitic structures. In the case of B microalloyed steel, the CCT curve is displaced to the right, increasing the hardenability. These results are associated with the ability of B atoms to segregate towards austenitic grain boundaries, which reduce the preferential sites for nucleation and development of F+P structures.

Mechanical properties and microstructure of low carbon ultra-high strength steels (UHSS) microalloyed with boron

ABSTRACTThe aim of this research work is to study the effect of boron addition on mechanical properties and microstructure of a new family of low carbon NiCrVCu advanced high strength steels (AHSS). Experimental steels are thermo-mechanically processed (TMP) (hot-rolled+quenched). Results show that the microstructure of these steels contains bainite and martensite, predominantly, which nucleate along prior austenite grain boundaries (GB). On the other hand, tensile tests reveal that the TMP steels have YS (0.2% offset) of 978 MPa, UTS of 1140 MPa and EL of 18%. On the basis of exhibited microstructure and mechanical properties, these experimental steels are classified as bainitic-martensitic complex phase (CP) advanced ultra-high strength steels (UHSS).

Carbide characterization in a Nb-microalloyed advanced ultrahigh strength steel after quenching–partitioning–tempering process

Based on the observations of scanning electron microscopy and transmission electron microscopy, four kinds of carbides were identified in a Nb-microalloyed steel after quenching–partitioning–tempering treatment. In addition to transitional epsilon carbide that usually forms in silicon-free carbon steel, other three types of niobium carbides (NbC) formed at various treatment stages respectively. They are incoherent NbC inclusion that nucleated at solidification mainly, fine NbC that nucleated in lath martensite at tempering stage and regular polygonal NbC that nucleated in austenite before quenching. Their formation mechanisms on steel were discussed briefly based on thermodynamics.

New advanced ultra-high strength steels for the automotive industry

A major challenge in car design is to meet safety standards that are continually being made tougher, whilst at the same time keeping the weight and cost of the car body to a minimum. With this in mind, SSAB Swedish Steel is continually adding to its range of ultra-high strength steels.