Ultra-high Strength Steel Research Articles

Tests, numerical study and design of Q1100 ultra-high strength steel welded I-section stub columns

An experimental and numerical study on the residual stresses and the cross-sectional behavior of Q1100 ultra-high strength steel welded I-sections is reported in this paper. The experimental tests included nine tensile coupons, eight stub column tests and measurements of the residual stresses and geometric imperfections of welded I-section stub columns. Based on the test results, finite element (FE) models were developed and validated, which were then employed in a parametric study to expand the available test data pool. The slenderness limits and design methods for high strength steel stub columns in the European, North American, Australian and Chinese design codes and the DSM method were examined by the test and FE data. Furthermore, a new modified DSM method offering improved accuracy for the design of Q1100 welded I-sections was proposed and validated via reliability analyses.

Evaluation of the Friction Coefficient for TRIP1000 Steel under Different Conditions of Lubrication, Contact Pressure, Sliding Speed and Working Temperature

The use of ultra-high-strength steels (UHSS) has been growing in recent years, mainly in the automotive industry. Since these steels have high strength and hardness, more applied stresses are required to deform them, probably also impacting friction behaviour. In this article, a variation in the process parameters commonly observed in sheet-metal forming, such as contact pressure, sliding speed, lubrication and working temperature was performed. The material used was TRIP1000. These process parameters were varied, aiming to investigate the friction-coefficient behaviour; however, it was observed that there were no significant variations, indicating that the steel hardness may have contributed to this. Another finding is that, even if the lubricant did not change the average value of the friction coefficient, it contributed to a more stable process, favouring the absence of premature wear of the tools.

Microstructures, heat treatments and mechanical properties of AerMet100 steel fabricated by hybrid directed energy deposition

As a kind of directed energy deposition (DED) method, wire and arc additive manufacturing (WAAM) is a solution for the short-process manufacturing of medium complex and integral parts. Ultrahigh-strength steel AerMet100 has been widely applied in large-scale aerospace structural parts like landing gear and the increasing demand of integrated design promotes the application of WAAM. The microstructure and mechanical properties of as-deposited AerMet100 steel are greatly affected by post-heat treatments, and thus researches on heat treatments are essential to the wider application of AerMet100 in the aerospace industry. In this paper, an AerMet100 steel sample was fabricated by WAAM with in situ microrolling (a hybrid DED method). Two heat treatment designs, based on standard and homogenization, were applied to the sample. The microstructures of the as-deposited and heat-treated specimens were examined with optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD), and the mechanical properties were examined using an electronic universal testing machine. The results indicate that austenitization is the main grain refinement mechanism of the as-deposited specimens, and a smaller layer height and an appropriately large heat input are beneficial for the refinement. Cellular segregation formed during solidification has severe heredity and strong effects on the microstructures of the nonhomogenized heat-treated group, causing lower mechanical properties and anisotropy. A special fracture mechanism related to segregation causes fracture morphology featuring parallel structures in both the non-homogenized X-direction tensile specimens and X-Y direction KIc specimens. Compared with the nonhomogenized heat-treated group, homogenization heat treatment basically eliminated cellular segregation and achieved a better combination of the tensile and KIc properties.

Seismic Performance of Steel tube-reinforced Steel Fiber high-strength Concrete Columns with ultra-high Strength Steel Bars

ABSTRACT Quasi-static tests of five full-scale steel tube-reinforced high-strength concrete (STRHSC) column with ultra-high strength steel bars (UHSS) were conducted. Due to the large elastic range of UHSS, the column can obtain obvious drift-hardening behavior to reduce residual deformation. The research results reveal that steel fiber-reinforced STRHSC columns with UHSS had superior damage control capability and reparability. According the damage evolution theoretical analysis and measured result, a prediction model of skeleton curve, residual deformation, cumulative energy dissipation and hysteretic curve of STRHSC columns with UHSS was proposed. The model was then verified by comparing prediction results with test results.

Analysis of Orientation Relationships, Carbon Partitioning and Strengthening Mechanism of a Novel Ultrahigh Strength Steel

Analysis of Orientation Relationships, Carbon Partitioning and Strengthening Mechanism of a Novel Ultrahigh Strength Steel

Influence of cooling rate during cryogenic treatment on the hierarchical microstructure and mechanical properties of M54 secondary hardening steel

Cryogenic treatment (CT) is an essential heat treatment process in improving the mechanical properties of secondary hardening ultra-high strength steel (SH-UHSS). The present study investigated the influence of cooling rate during CT on the hierarchical microstructure and strength-toughness of M54 SH-UHSS. Increasing cooling rate can synchronously refine martensite matrix and M2C-type (M = Mo, Cr, W, V) carbides, while there are the most refined martensite blocks and highest-density precipitation of the carbides at cooling rate of 3 °C·min-1. Further increased cooling rate weakens the effect of CT on the refinement and precipitation of martensite matrix. The refinement is related to the high-level segregation of carbon atoms and favorable equilibrium concentration of vacancies during CT and pinning effect of carbides on mobile dislocation during tempering. Besides, uniform carbon clusters and high-nucleation rate by the refined martensite matrix and high-density of dislocations mostly contribute to the above beneficial precipitation of carbides. Precipitation and martensitic matrix strengthening mostly contribute to the ultra-high strength (yield strength of 1730 MPa, ultimate tensile strength of 2018 MPa) at the cooling rate of 3 °C·min-1; meanwhile, the refined blocks as "effective" controlling unit is identified to be the major toughening mechanism contributing to a desirable impact toughness (V-notched Charpy impact energy of 30 J). This study would be instructive for processing the engineering components with large cross-section.

Effect of Ni addition upon microstructure and mechanical properties of hot isostatic pressed 30CrMnSiNi2A ultrahigh strength steel

Hot isostatic pressing is an advanced powder metallurgical technology for the production of complex and special-shaped components made of ultrahigh strength steel. However, due to the low hardenability of 30CrMnSiNi2A steel, it is difficult to achieve the expected martensitic structure in the sintering process. In this study, the hardenability of the alloy is improved by adding Ni into 30CrMnSiNi2A steel powder. The microstructure, tensile properties and fracture behavior of the sintered 30CrMnSiNi2A-xNi steels (x = 0, 1.5 wt%, and 3.0 wt%) are systematically investigated. Experimental results show that with the increase in Ni addition, the matrix structure changes from granular bainite (0 wt% Ni) to lath bainite (1.5 wt% Ni) and lath martensite (3.0 wt% Ni); and the ultimate tensile strength increases from 1312 MPa to 1466 MPa and 1747 MPa. The improvement in strength is mainly related to the solid solution strengthening of C and Ni atoms in the matrix structure. In addition, the increase in dislocation density and grain boundary density in the matrix structure also promotes the improvement in strength, according to the kernel average misorientation and grain boundary analysis of EBSD. After the addition of Ni powder, the fracture mode changes from the transgranular quasi-cleavage to the interparticle debonding. This is mainly due to the increase in the oxide density at the particle boundaries and the matrix strength.

Fatigue performance of notched and hot-dip galvanized laser and mechanically cut S960 steel components considering local defects with the theory of critical distances

Experimental fatigue tests were performed for a S960 steel grade, including hot-dip galvanized (HDG) round base material specimens, and laser cut, and machined notched component-sized specimens made of t = 6 mm S960 ultra-high-strength steel (UHSS) plates. Cracking after the HDG was found to have a major influence on fatigue strength and thus reducing the effect of surface quality on the fatigue performance. Design guidelines for notched HDG components are proposed, and HDG with UHSSs was found suitable for structures with geometrical notches. Multiparametric TCD-based 4R method application was introduced, and it was found to be applicable for the fatigue strength assessment of structural details with initial cracks.

Study on high temperature flow behavior of BR1500HS ultra-high strength steel

Study on high temperature flow behavior of BR1500HS ultra-high strength steel

Mean-stress sensitivity of an ultrahigh-strength steel under uniaxial and torsional high and very high cycle fatigue loading.

The influence of load ratio on the high and very high cycle fatigue (VHCF) strength of Ck45M steel processed by thermomechanical rolling integrated direct quenching was investigated. Ultrasonic fatigue tests were performed under uniaxial and torsional loading at load ratios of R = -1, 0.05, 0.3, and 0.5 with smooth specimens and specimens containing artificially introduced defects. Up to 2 × 105 cycles, failure originated from surface aluminate inclusions and pits under both loading conditions. The prevailing fracture mechanisms in the VHCF regime were interior crack initiation under uniaxial loading and surface shear crack initiation under torsional loading. The mean-stress sensitivity and the fatigue strength were evaluated using fracture mechanics approaches. Equal fatigue limits for uniaxial and torsional loading were determined considering the size of crack initiating defects and the appropriate threshold condition for Mode-I crack growth. Furthermore, the mean-stress sensitivity is independent of loading condition and can be expressed by and .

Evaluation of Fatigue Life of Ultra-High-Strength Steel under Stress Corrosion Environment

Ultra-high-strength steel (UHSS) structures are exposed to corrosive environments during service, and hydrogen-assisted cracking (HAC) may occur owing to stress corrosion cracking and hydrogen embrittlement. In this study, the HAC threshold stress intensity factor and fatigue life of UHSS steel were evaluated by applying stress in a corrosive environment to prevent structural fracture. For specimen with semicircular slits by electric discharge machining, fatigue limit was obtained by static fatigue test under corrosive environment. The fatigue limit of the crack specimen was evaluated by the fatigue limit of the experiment and HAC threshold stress intensity factor, and comparative evaluation was performed. On the surface of cracks, grain boundaries were embrittled by corrosion, and grains were clearly observed. Meanwhile, cracks in the surface direction propagated slightly, unlike cracks in the depth direction. The static fatigue limit of UHSS (SKD11:HV670) was determined to be 400 MPa, and the fatigue limit of the crack specimen could be evaluated. The experimental results agreed well with the evaluation results.

S960 ultra-high strength steel slender welded I-section beam–columns: Testing, numerical modelling and design

This paper presents experimental and numerical investigations of the buckling behaviour and resistance of S960 ultra-high strength steel slender welded I-section beam–columns under combined compression and minor-axis bending moment. An experimental programme, including initial geometric imperfection measurements and eight beam–column tests, was firstly conducted. The test procedures, setups and results were fully reported. Following the experimental programme, a numerical modelling programme was carried out, with finite element models initially developed and validated against the test results and then employed to conduct parametric studies to generate further numerical data. On the basis of the test and numerical data, the relevant design interaction curves, as provided in the European code, Australian standard and American specification, were evaluated for their applicability to S960 ultra-high strength steel slender welded I-section beam–columns. The evaluation results revealed that all three codified design interaction curves lead to conservative resistance predictions, mainly owing to the conservative end points (i.e. column buckling strength and cross-section minor-axis bending resistance). Finally, a revised Eurocode design interaction curve, with improved compression end point determined from more accurate column buckling curve and bending end point determined from the direct strength method, was finally proposed, and shown to result in more accurate resistance predictions for S960 ultra-high strength steel slender welded I-section beam–columns than the codified design interaction curves.

Influence of Oxygen Content on the Microstructure and Mechanical Properties of Hot Isostatically Pressed 30CrMnSiNi2A Ultra-High Strength Steel

Influence of Oxygen Content on the Microstructure and Mechanical Properties of Hot Isostatically Pressed 30CrMnSiNi2A Ultra-High Strength Steel

Improvement of Resistance to Hydrogen Embrittlement for 1300MPa Ultra-high Strength Steel by Introducing Nano-sized Cu-rich Phase Precipitation

Improvement of Resistance to Hydrogen Embrittlement for 1300MPa Ultra-high Strength Steel by Introducing Nano-sized Cu-rich Phase Precipitation

Simultaneous increase of ultimate tensile strength and uniform elongation of 30Si2MnCrMoVE ultra-high strength steel by hot deformation direct quenching and partitioning

In order to further improve the combination of strength and ductility for 30Si2MnCrMoVE ultra-high strength steel (UHSS), a thermomechanical treatment, named deformation direct quenching and partitioning (DQ&P), was carried out on it. The microstructure evolutions and mechanical properties of 30Si2MnCrMoVE UHSS treated by DQ&P were compared with those treated by conventional off-line quenching and tempering (Q&T) through various microscopic characterization methods and uniaxial tensile tests. Moreover, the effects of austempering temperature on the microstructure evolutions and mechanical properties of the studied steel after DQ&P process were also investigated. The results show that the DQ&P samples, although lower or slightly lower yield strength, possess significantly higher ultimate tensile strength due to higher work-hardening rate caused by high density dislocations and fine dispersed carbides. Moreover, DQ&P310 sample exhibit apparently higher uniform elongation than Q&T sample despite of comparable content of retained austenite due to more prone to transformation induced plasticity (TRIP) effect. The DQ&P process can increase the ultimate tensile strength by 267 MPã423 MPa and the uniform elongation by 78%∼85%, respectively, as compared to Q&T process, which indicates that DQ&P process may be a promising thermomechanical treatment method to improve the comprehensive mechanical properties of the studied steel. The strengths and elongations of DQ&P samples do not vary monotonously with the increase of austempering temperature, which are both closely related to the content of retained austenite.

Phase transformation and mechanical properties of ultrahigh strength steels under continuous cooling conditions

The present work deals with phase transformation mechanical behavior and of newly developed ultrahigh strength steels. Bainitic and martensitic phase transformations have been studied in detail with respect to their transformation temperature range and microstructural evidences observed under various continuous cooling conditions. The various critical temperatures of Ac1, Ac3, Bs, Ms, and TNR of the investigated steels were initially calculated using numerical equations and validated with experimental results. The dilatometric tests consisted under different cooling rates from 0.1 to 100 °C/s and the respective continuous cooling transformation (CCT) diagrams have been determined. The microstructural features were revealed using optical and different electron microscopy techniques. A strong dependency of cooling rate on microstructure and mechanical properties has been found. The present steels have exhibited ultrahigh strength (1400–2000 MPa) with reasonable ductility (6–13% total elongation), which is attributed to combination of mixed microstructure comprising fine bainite and lath martensite along with thin film-like retained austenite. Fine precipitates of (Ti, Nb)C, Mo2C and/or Cu contribute to strengthening effect by increasing the hardness and strength values for a particular cooling rate. It has been witnessed that the hardness, yield strength and ultimate tensile strength have been increased with the rise of both cooling rate and carbon contents.

Influence of heat treatment (routes) on the microstructure and mechanical properties of 300M ultra high strength steel

Influence of heat treatment (routes) on the microstructure and mechanical properties of 300M ultra high strength steel

Tensile and Pull-Out Behavior of Steel Reinforced Grout Connectors

Mortar-based composites are an emerging technology for the repair and strengthening of reinforced concrete and masonry structures. In most cases, the effectiveness of the retrofitting work relies on the substrate-to-composite bond capacity but in some applications, connectors are also used to prevent debonding and improve the performance of the retrofitted structure. Indeed, the use of connectors is recommended by design guidelines and suppliers are required to test them for acceptance. The paper presents a laboratory investigation on steel reinforced grout connectors, made by rolling ultra-high tensile strength steel textiles, comprising either galvanized or stainless-steel micro cords. Tensile tests were first carried out for mechanical characterization. Pull-out tests were then performed on connectors installed in holes drilled in wall panels and injected with either cement or lime mortars. Concrete, tuff masonry, brickwork and limestone masonry were used as substrate materials. Test results are commented to analyze the effect of textile rolling on tensile response, of textile and matrix properties on pull-out strength and failure mode, as well as to highlight their significance for design purposes.

Experimental study on seismic performance of resilient recycled aggregate concrete shear walls

A type of recycled aggregate concrete shear wall was proposed in this work, which used the painted ultra-high-strength steel (UHSS) reinforcements with low bond strength and high yield strength in the boundary elements. For investigating the seismic resistance and resilient performance of this shear wall, six shear walls with the shear-to-span ratio of 1.5 were fabricated and tested under the cyclic loads. The variables studied in this paper were the types of the longitudinal reinforcement in the boundary elements, the existence of steel fibers, the presence of the concealed bracings (X-shaped) and the different axial compression ratios. The test results indicated that specimens with the UHSS reinforcement in the boundary elements decreased the residual deformation substantially and improved the resilient properties. The higher axial compression ratio enhanced lateral force resistance of shear wall but aggravated concrete damage which was adverse to the resilience. In addition, the utilization of steel fibers in concrete significantly reduced the crack width. The shear wall including concealed bracing not only showed much higher lateral force resistance, but also had desirable resilient performance under the large deformation. Finally, a simplified calculation model for such shear walls was proposed on the basis of experiment. And the good accuracy of the calculation model was verified by comparing the calculation results and the measured results.

Process Stability and Application of 1900 MPa Grade Press Hardening Steel with reduced Hydrogen Susceptibility

While 22MnB5 with aluminium silicon (AS) coating was established as the quasi-standard in press hardening many years ago, the need for steels that offer even higher strength is growing. These needs include corrosion protection of the automotive component and scale prevention during processing in hot forming lines. However, the processing of AS coated ultra-high strength steels involves demands for the furnace atmosphere. Reducing the hydrogen susceptibility and increasing the material strength at the same time was the driving force for the development of MBW 1900 + AS Pro. The paper focuses on the process stability of this steel concerning furnace and stamping parameters as well as the hydrogen induced cracking resistance under different dew points in the furnace atmosphere. In addition, the potential by means of different partial press hardening processes is presented. With tailored tempering, locally heated tools can be used to achieve soft areas in the part. Furthermore, the application potential of MBW 1900 + AS Pro in tailor welded blanks is shown.