Development Of High-strength Steels Research Articles

Excellent low-temperature toughness of 1 GPa grade microlaminated 5Mn steel containing retained austenite

The development of high-strength steels with excellent low-temperature toughness continues to present significant challenges. Delamination toughening and retained austenite (RA) toughening are effective methods for improving the low-temperature toughness of steels. In the present study, medium manganese steel with a microlaminated microstructure containing blocky RA is prepared by intercritical rolling (IR). A comparative steel with a microstructure containing annealed martensite and film-like and blocky RA is generated by hot rolling and intercritical annealing (HRIA). The different microstructure morphologies lead to different fracture characteristics. The IR steel plate exhibited a delaminated fracture that changes the crack propagation path, inducing large-scale and severe plastic deformation in the area below the delaminated crack in the entire temperature range (RT ∼ −196 °C). The Charpy impact V-notch absorbed energy (CVN) is as high as 450 ± 7 J at −60 °C. Moreover, the fracture behavior of the HRIA steel was similar to that of steels with equiaxed microstructures; ductile fracture takes place (above −40 °C) first, followed by brittle fracture, and the CVN is only 150 ± 83 J at −60 °C. The RA contents of the IR and HRIA steels at RT are 25.8% and 22.3%, respectively. Upon impact at −60 °C, the RA content becomes 0 and 4% in the IR and HRIA steels, respectively. Finally, a toughening mechanism is established for IR steel from the perspectives of crack propagation and microstructure evolution.

Analysis of grain-boundary segregation of hydrogen in bcc-Fe polycrystals via a nano-polycrystalline grain-boundary model

Hydrogen embrittlement caused by hydrogen segregation at grain boundaries (GBs) is the most serious issue in the development of high-strength steels, but the mechanisms behind this process are still not well understood. The GB segregation behavior of hydrogen in body-centered cubic (bcc)-Fe polycrystals was comprehensively analyzed based on the interatomic potentials derived from first-principles calculations. Considering that the atomic structure of GBs is almost independent of the grain size, the GBs in polycrystals were modeled as nano-polycrystalline GBs with random orientations. The segregation energies of hydrogen for ∼17 million interstitial sites in this GB model were calculated. From these segregation energies, the effective segregation energy for the polycrystalline GB at thermal equilibrium under various temperature and hydrogen content conditions were determined, and the validity of the calculation method was verified by comparing the results with experimental data. The relationship between the segregation energy of hydrogen at each segregation site and the surrounding local atomic environment was used to identify the major hydrogen segregation sites at the atomic level, and the changes in the crystal structure near the GB that dominated segregation were clarified. The effective segregation energies at the polycrystalline GBs were in the range of −0.48 to −0.42 eV, which are in good agreement with the experimentally reported binding energy of hydrogen at GBs of bcc-Fe polycrystals (−0.52 eV). The major hydrogen segregation sites were octahedral sites with Voronoi volumes larger than 7.0 Å3, and the segregation energy was mainly due to the uniaxially distorted crystal structure in the short-axis direction of octahedral sites. Our findings and the developed calculation method contribute to the understanding of the hydrogen segregation behavior and hydrogen embrittlement mechanism in polycrystalline metallic materials.

Response of hydrogen diffusion and hydrogen embrittlement to Cu addition in low carbon low alloy steel

Hydrogen embrittlement (HE) has arisen as a critical issue for the development of high strength steel. The austenite and Cu-rich precipitates can provide stable trapping sites for diffusible hydrogen, and thus improved the resistance to HE. But the research on the synergistic effect of Cu and austenite on HE in low carbon low alloy steel has not been actively reported. In this study, four steels with varying Cu addition (0-3 wt%) were fabricated, and their resistance to HE was evaluated. The role of Cu addition was investigated by the ductile-brittle transition temperature (DBTT) value obtained from Charpy impact test and the loss of elongation measured from slow-strain-rate tensile (SSRT) test after hydrogen charging. Also, the hydrogen diffusivity and concentration were evaluated by electrochemical hydrogen permeation test. Cu addition resulted in a first decrease and subsequent increase of effective diffusion coefficient (D eff ), elongation loss and DBTT, and the minimum value was achieved in steel with 1.5 wt% Cu addition, indicating a higher resistance to HE. The unraveled mechanism was that Cu addition increased the content of retained austenite, which impeded the diffusion of hydrogen during deformation. But the stability of retained austenite decreased when the content of retained austenite reached a high value, which led to martensitic transformation and accordingly increased HE susceptibility. Moreover, Cu addition promoted the formation of Cu-rich precipitates in which Cu cluster/bcc Cu-rich precipitate coherent/semi-coherent with matrix can be strong trapping sites for hydrogen, and fcc Cu-rich precipitate acted as weak trapping sites for hydrogen. In addition, a higher density of dislocations was found in the steel with 1.5 wt% Cu addition, which may contribute to lower D eff and higher density of hydrogen traps (N T ). The present work suggested that Cu addition can enhance the resistance to HE during tensile and impact processes for the design of high strength steel. • Cu addition resulted in a first decrease and subsequent increase in the effective diffusion coefficient (D eff ) of hydrogen. • Cu addition increased the elongation rate and decreased the ductile-brittle transition temperature (DBTT) of the steels after hydrogen embrittlement. • The resistance to hydrogen embrittlement during tensile and impact process was determined by retained austenite, Cu-rich precipitates and dislocations in the microstructure. • The resistance to hydrogen embrittlement varied with the content and stability of austenite and the structure of Cu-rich precipitates.

Effect of Mo addition on hydrogen segregation at α-Fe grain boundaries: A first-principles investigation of the mechanism by which Mo addition improves hydrogen embrittlement resistance in high-strength steels

The development of high-strength steels is essential for realizing a decarbonized society; however, to achieve this, the problem of hydrogen embrittlement must be solved. The effectiveness of adding Mo has been demonstrated to address this issue, but its underlying mechanism has not yet been clarified. To investigate this mechanism, we developed a calculation method to evaluate the effect of adding alloying elements on the grain boundary segregation of H using parameters calculated via first-principles calculations. Accordingly, the segregation energies of H with and without Mo segregation near the grain boundary were calculated. Based on the obtained segregation energies, the concentration of H at the grain boundary was calculated for a given amount of Mo under the heat treatment conditions normally assumed. The results showed that a small amount of Mo segregates to α-Fe grain boundaries and significantly reduces the grain boundary concentration of H owing to repulsive interactions between Mo and H. This indicates that Mo addition improves the resistance to hydrogen embrittlement by reducing the grain boundary H concentration. The physical origin of these repulsive interactions was further investigated by decomposing them into mechanical and chemical contributions; we found that these repulsive interactions are mainly caused by the chemical contribution. Furthermore, the crystal orbital Hamiltonian population indicated that the chemical contribution can be attributed to the large core repulsion between Mo–H at α-Fe grain boundaries (in comparison to that between Fe–H) owing to the larger atomic orbital of Mo. The proposed calculation method as well as the information obtained from our evaluations can help in the development of high-strength steels with excellent resistance to hydrogen embrittlement and in the improvement the resistance to hydrogen embrittlement of various metallic materials.

Comparison and Analysis of Steel Frame Based on High Strength Column and Normal Strength Column

The anti-seismic performance of high strength steel has restricted its industrialization in civil buildings. In order to study the influence of high strength steel column on frame structure, three models are designed through MIDAS/GEN finite element software. By comparing the seismic performance and economic performance of the three models, the three different structures are comprehensively evaluated to provide some references for the development of high strength steel in steel structure.

The Effect of Annealing Temperature on Microstructure and Mechanical Properties of Lightweight Steel with Increased Aluminium Content

A demand for enlightening of constructions in an automotive industry resulted in an intensive development of high strength steels and in attempts to decrease the weight of the steel by intensive alloying by lighter elements. This work used chemical concept of AHSS (advanced high strength steel) TRIP (transformation induced plasticity) steel with 0.2%C and micro-alloyed by 0.06%Nb and increased manganese content to 4% and aluminium to 6.5% to produce lightweight steel, which was cast and re-forged and subsequently annealed at various temperatures in the range of 300°C - 800°C. The resulting microstructures were analysed by light microscopy, laser scanning confocal microscopy, scanning electron microscopy and X-ray diffraction phase analysis and mechanical properties were measured by a tensile test. Tensile strengths in the region of 600 MPa - 757 MPa and total elongations around 20% were obtained for annealed samples.

Review on High Strength Steel Bolted End-Plate Connections

Past three decades have seen the rapid development of high strength steel (HSS) in its application in structural engineering. However, so far the mechanical performance of a HSS beam-to-column connection has not been systematically studied, especially for bolted end-plate connections, the commonly employed beam-to-column connections in steel structures, which could restrict the application of HSS. Therefore, this paper aims to represent the basic methods, current achievements, recent applications, and the existing problems that lie in the way. In doing so, this paper is composed of three parts, experimental results, numerical analysis as well as component method. At the end, this paper indicates that future investigation should be based upon experimental analysis and proper finite element modeling, to verify a numerical model and to refine design standards.

Fracture Toughness Characterization of High-Pressure Pipe Girth Welds Using Single-Edge Notched Tension [SE(T)] Specimens

Abstract The safety and reliability of large-diameter pipelines for the transport of fluid hydrocarbons is being improved by the development of high-strength steels, advanced weld technologies, and strain-based design (SBD) methodologies. In SBD, a limit is imposed on the applied strains rather than the applied stresses. For high-pressure pipelines, SBD requires an assured strength overmatch for the weld metal as compared to the base material, in order to avoid strain localization in the weldment during service. Achieving the required level of strength overmatch, as well as acceptable ductility and low-temperature fracture toughness, is a challenge as the pipe strength increases. Published studies show that low constraint geometries such as single-edge tension [SE(T)] or shallow-notched single-edge bend [SE(B)] specimens represent a better match to the constraint conditions of surface-breaking circumferential cracks in large-diameter pipelines during service (Shen, G., Bouchard, R., Gianetto, J. A., and Tyson, W. R., “Fracture Toughness Evaluation of High Strength Steel Pipe,” Proceedings of PVP2008, ASME Pressure Vessel and Piping Division Conference, Chicago, IL, July 27–31, ASME, New York, 2008). However, the SE(T) geometry is not included in any of the most widely used elastic-plastic fracture mechanics (EPFM) test standards. A procedure has been developed for performing and analyzing SE(T) toughness tests using a single-specimen technique that includes formulas for calculating the J-integral and crack-tip opening displacement, as well as for estimating crack size using rotation-corrected elastic unloading compliance. Here, crack-resistance curves and critical toughness values obtained from shallow-crack SE(T) specimens (a0/W ≈ 0.25) are compared to shallow-crack (a0/W ≈ 0.25) SE(B) specimens. We believe that the SE(T) methodology is mature enough to be considered for inclusion in future revisions of EPFM standards such as ASTM E1820 and ISO 12135, although additional work is needed to establish validity limits for SE(T) specimens.

The influence of robotic MAG process welding parameters

In the modern manufacturing sector, new materials and current fabrication technologies force a critical re-examination of all conventional knowledge. Increasing production and reducing waste remain fundamental objectives for every business: the trend is moving towards increasingly reduced labour, which is gradually being replaced by automation. On the other hand, especially in such a fundamental sector as the automotive industry, there is a prime requirement: reduced vehicle weight, due to the incessant demand for energy and cost savings, in addition to meeting commitments relating to harmful emissions and environmental protection. The metallurgical development of high-strength steels has allowed the use of new materials with increasingly enhanced mechanical characteristics, benefiting mankind with increased safety and the ability to absorb ever greater quantities of energy without cracking. To analyse the problems associated with the types of welding used with increasing frequency in the automobile sector, research has been focussed on joints made between conventional materials such as S355 steel and high-strength Dual Phase steels. This study has attempted to highlight the use of a welding robot in a mass production setting, and to study the best welding parameters when not working with components with the same metallurgical characteristics. Furthermore, the ability to obtain good quality welds when there are geometrical problems, such as misalignments between the pieces to be welded, has been evaluated.

Research Progress on the Mechanical Property of High Strength Structural Steels

High strength and high performance steels used in structural engineering have been developed, but the application is constrained and the design work changes as a result of the different material property. In order to study high strength steel material property features, the production process, steel grade and code limit requirement were reviewed and discussed. Some important mechanical property indexes including the stress-strain relationship, yield-tensile strength ratio (i.e. Y/T ratio), ductility and toughness, were analyzed based on a large amount of tension coupon test data reported in the available literature. The stress-strain relationship model used for finite element analysis was proposed. It is concluded that high strength structural steels have excellent toughness but lager Y/T ratio and lower elongation compared with those of traditional steels, some of which even exceed the code limit values. It is necessary to do deep research on the relationship between the steel mechanical property indexes and structure safety and to make further development of high strength steels. This research work is helpful to comprehend the material property of high strength structural steels and relevant code limit requirements.

Application of thermodynamics and diffusion in solids to the development of high strength steels for the Automotive Industry

This paper presents some applications of thermodynamics and diffusion in solids to the development of physical models applied to high strength steels production for the Automotive Industry. The scale of modeling is chosen to be the microstructural grain. The description of nucleation, interfacial conditions, diffusion controlled growth makes it possible to follow the process of microstructure formation, the evolution of each steel constituent and finally to describe the produced steel microstructure. In this paper, we propose a model that predicts the kinetics of cementite dissolution and austenite growth during heating and soaking. It is based on solving the mass balance and diffusion equations written with gradients of chemical potentials as driving forces. This model is applicable to multi component systems and a good agreement between the calculations and experimental data was obtained. Anwendung der Thermodynamik und Diffusion in Festkörpern zur Entwicklung hochfester Stähle in der Automobilindustrie Im Rahmen dieser Arbeit wird die Anwendung der Thermodynamik und Diffusion in Festkörpern zur Entwicklung hochfester Stähle in der Automobilindustrie vorgestellt. Die Modellierung erfolgt auf der mikrostrukturellen Ebene der Körner. Die Beschreibung der Keimbildung, der Bedingungen an den Grenzflächen und des diffusionskontrollierten Wachstums ermöglicht dem Prozess der Mikrostrukturbildung und der Entwicklung jeder Stahlkomponente zu folgen und schließlich die entstandene Mikrostruktur des Stahls zu beschreiben. In dieser Arbeit wird ein Modell, zur Vorhersage der Zementitauflösung und des Austenitwachstums während des Erhitzens und des Abschreckens, vorgeschlagen. Es basiert auf der Lösung der Massenbilanz- und Diffusionsgleichungen, die mit Gradienten des chemischen Potenzials als treibende Kraft formuliert werden. Das Modell kann auf Multikomponentensysteme angewendet werden und eine gute Übereinstimmung zwischen Berechnung und Experiment wurde erzielt.

Fatigue Strength Improvement of Bridge Girders by Ultrasonic Impact Treatment (UIT)

Over the past four years, extensive research has been initiated on the application of ultrasonic impact treatment (UIT) for fatigue enhancement of fracture-critical details in steel bridges. The research is particularly relevant in light of the development of high strength steels and their use within the bridge industry. The main investigation involved fatigue tests on full-size fabricated and rolled ASTM A709 GR 50W steel girders with welded attachments (stiffeners and cover plates). These enabled the influence of weld detail design on the benefit of UIT to be investigated. At the same time, a series of fatigue tests was also carried out, aimed at the comparative evaluation of the effectiveness of various post-weld fatigue improvement methods. The results obtained to-date in these investigations are presented in this paper. Specific emphasis is focused on weld design options and the benefits achieved by ultrasonic impact treatment. The fatigue test results obtained to-date from the research are presented in this paper. Conclusions are drawn regarding the improvement in fatigue performance that can be achieved with UIT for consideration in the design of welded bridge structures as well as maintenance and repair procedures.

溶接部高疲労強度鋼板の開発

溶接部高疲労強度鋼板の開発

Development of high strength steels by new alloying and sintering techniques : E.Ernst et al. (Sintermetallwerk Krebsoge GmbH, Radevormwald, Germany.)

Development of high strength steels by new alloying and sintering techniques : E.Ernst et al. (Sintermetallwerk Krebsoge GmbH, Radevormwald, Germany.)

自動車用高強度高成形性鋼板の開発

自動車用高強度高成形性鋼板の開発

軽量化と低コスト化を両立する高強度コネクティングロッド用鋼の開発

軽量化と低コスト化を両立する高強度コネクティングロッド用鋼の開発

Relationship between the strength and sensitivity to stress concentrations of martensitic Cr?Co?Ni?Mo steel

1. Martensitic stainless steels of the 9Cr−14Co−Ni and 9Cr−14Co−6Ni−Mo types are promising materials for the development of high-strength steels for cryogenic technology. The presence of nickel and molybdenum (the latter up to 3%) simultaneously increases the strength and lowers the sensitivity to stress concentrations at 20 and −196°. 2. An ultimate strength of 150 kgf/mm2 is the level at which one observes an increase in the sensitivity to stress concentrations at −196° for steels of the 9Cr−14Co−6Ni−Mo type, manifest in lower values of σbCr, KIc, andanIV. 3. Stainless steel of the Cr−14Co−6Ni−Mo system has high sensitivity to cracks with σb>150 kgf/mm2. 4. Qualitative agreements between the variations of σbCr, KIc, andanIV with the strength level and chemical composition of the steel were found.

A study of the system Fe-V-C-N with a view to the precipitation hardening process

The paper deals with the analysis of the precipitate composition and the precipitation mechanism in the system Fe-V-C-N. The aim of this paper is also to discuss the way of vanadium carbide hardening in commercial type CrMoV steel. Further, the paper quotes data on hexagonal vanadium nitride V2N which becomes an important element in the development of high-strength steels.