Mechanical Properties Of High-strength Steel Research Articles

Effect of tempering temperatures on microstructure and mechanical properties of high-strength steel

Effect of tempering temperatures on microstructure and mechanical properties of high-strength steel

Microstructure and mechanical properties of the laser welded air-hardening steel joint

The decrease in mechanical properties of high-strength steel after welding is an important issue affecting the wide application of high-strength steel. Air-hardening steel is a high-strength steel suitable for lower body structural parts such as subframes. Its application process involves welding, hot forming and other processes. The present work investigates the microstructure and mechanical properties of the air-hardening steel laser welded joint that is air-cooled after hot forming in the two-phase zone (800 °C). The microstructure was characterized by electron backscattered diffraction (EBSD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that during hot forming, the welded joint transforms from martensite to ferrite and acicular martensite, and the base metal transforms from ferrite to polygonal martensite and ferrite. The difference in martensite morphology between the welded joint and the base metal is attributed to the nucleation positions of austenite. The structural evolution of the welded joint and the base metal is accompanied with the annihilation and reproduction of dislocations, which results in significant changes in hardness. The hardness value dropped from the highest 430 HV to 271 HV in the welded joint, while increased from the lowest 184 HV to 203 HV in the base metal. After hot forming, the tensile strength of the welded sample is reduced by only 36 MPa, and the total elongation is slightly decreased by about 1.5% compared with the unwelded sample. The welded joint and the base metal have similar plastic deformation capabilities, since the acicular martensite in the welded joint displays good plastic deformation ability, and the dislocation density of the welded joint and the base metal is similar. Overall, the microstructure and dislocation density of the air-hardening steel welded joint after hot forming are similar to those of the base metal, which is responsible for the good mechanical properties of air-hardening steel welded joint.

Mechanical performances of thin-walled high-strength concrete-filled steel tube square columns with high-strength reinforced cages under biaxial eccentric compression

In the current research that both steel tube and concrete use high-strength materials, there are few studies on the small wall thickness of steel tube exceeding the standard. In order to make full use of the mechanical properties of high-strength steel and enhance its restraint ability on concrete, this paper designs a steel cage dominated by transverse steel bars. A total of 15 square high-strength steel tube high-strength concrete medium-long column test pieces are tested and studied. The main parameters of the specimens are slenderness ratio, loading eccentricity and the form of internal steel reinforcement cage. After the loading test, according to the test results, the failure mode of the specimen is obtained and the mechanical properties of middle-long concrete-filled steel tubular columns are analyzed. The research results show that the ultra-thin high-strength steel pipe can restrain concrete well, but with the increase of eccentricity, its restraint capacity is slightly insufficient; the built-in steel cage can slow down the bulging of the specimen and help the steel pipe to further strengthen the restraint on concrete, but the steel cage as an indirect constraint cannot avoid the final bulging damage. Due to the existing formula overestimating the effect of ultra-thin steel tubes, this paper proposes a calculation formula for the axial compression of thin-walled high-strength concrete-filled steel tubes with steel cage short columns. Based on this method, the bearing capacity of medium-long columns under biaxial eccentric compression is calculated with good accuracy.

Stresses in repair welding of high-strength steels—part 1: restraint and cold cracking risk

The sustainable and resource-efficient production of wind energy plants requires the use of modern high-strength fine-grain structural steels. This applies to both foundation and erection structures, like mobile or ship cranes. During the assembly of steel structures, unacceptable defects can occasionally be found in the weld area. In most cases, the economical solution would be local thermal gouging of the affected areas and re-welding. Due to the high shrinkage restraint of the joint groove in the overall structure, the superposition of global and local welding-induced stresses may lead to crack formation and component failure, particularly in interaction with the degradation of the microstructure and mechanical properties of high-strength steels during the repair process. However, manufacturers hardly have any information about these issues and there is a lack of recommendations and guidelines to take these safety-relevant aspects into account in adequate repair concepts. The aim of this research is to derive recommendations for repair concepts appropriate to the stresses and materials involved providing a basis for standards and guidelines to avoid cold cracking, damage and expensive reworking especially for high-strength steels. Part 1 of this study involves systematic investigations of influences of shrinkage restraint during repair welding of two high-strength steels S500MLO for offshore application and S960QL for mobile crane structures. The quantification of the shrinkage restraint of repair weld joints was achieved by means of experimental and numerical restraint intensity analysis. In welding experiments with self-restrained slot specimens, restraint intensity and introduction of hydrogen via the welding arc using anti spatter spray were varied systematically to analyse the effect on welding result, residual stresses and cold cracking. It could be shown that increasing restraint intensities result in significantly higher transverse residual stress levels. In the case of hydrogen introduction S500MLO showed no cold cracking independent of the restraint conditions. However, S960QL was found to be considerably cold cracking sensitive if hydrogen is introduced. With increasing restraint intensity length and number of cold cracks increases significantly. Part 2 [1] of this study is focussed on microstructure and residual stresses due to gouging and stress optimization via adequate heat control parameters in repair welding.

Effect of Segregation Band on the Microstructure and Properties of a Wind Power Steel before and after Simulated Welding

This article uses scanning electron microscopy (SEM) and electron back-scattering diffraction (EBSD) to study the effect of C and Mn segregation on the microstructure and mechanical properties of high-strength steel with 20 mm thickness used for wind power before and after simulated welding. A Gleeble-3500 (GTC, Dynamic Systems Inc., Poestenkill, NY, USA) was used to study the microstructure evolution of the simulated coarse-grained heat-affected zone (CGHAZ) of experimental steel under different welding heat inputs (10, 14, 20, 30 and 50 kJ/cm) and its relationship with low-temperature impact toughness (−60 °C). The results indicate that alloy element segregation, especially Mn segregation, significantly affects the impact toughness scatter of the steel matrix, as it induces the formation of low-temperature martensite or hard phase, such as M/A (martensite/austenite) constituent. In addition, segregation also reduces the low-temperature impact toughness of the simulated welding samples and increases the fluctuation range. For high-strength steel with yield strength higher than 460 MPa used for wind power generation, there is an optimal welding heat input (~20 kJ/cm), which enables the simulated coarse-grained heat-affected zone (CGHAZ) to obtain the highest impact toughness due to the formation of lath bainite (LB) and the finest crystallographic block units. Excessive or insufficient heat input can induce the formation of coarse granular bainite (GB) or lath martensite (LM), leading to a larger size of crystallographic block units, reducing the hindering effect of brittle crack propagation and deteriorating low-temperature impact toughness.

First principles and molecular dynamics simulations of effect of dopants on properties of high strength steel for hydrogen storage vessels

High-pressure gaseous hydrogen storage is an important way of hydrogen energy storage and transport at present, while high-strength steel material is one of the main materials used for hydrogen storage vessels. However, their internal doping elements and inherent defects often lead their mechanical properties to decrease, thus reducing the pressure-bearing capability and storage life of the vessel. At present, the mechanism of doping elements influencing the mechanical properties of high-strength steels is still unclear. In this work, a first-principles approach is used to study the influence of elemental doping (Cr, Mn, Mo, As, Sb, Bi, Sn, Pb) on the mechanical properties of Fe single crystals and Fe-C systems. The results show that among the above elements, Mn doping can increase the elastic modulus, bulk modulus, and shear modulus compared with those of pure Fe, while the doping by remaining elements will reduce the three moduli above, with the non-transition metal elements having a greater effect on the three moduli than the transition metal elements. Electronic structure analysis shows that the transition metal elements have better compatibility with the Fe lattice. Molecular dynamics results further show that the injection of H atoms significantly disrupts the lattice ordering of the Fe polycrystalline doped by C, Cr, and Mn elements, while the doping of Cr elements can significantly enhance the dislocation density of the system. The effects of doping elements on the mechanical properties of single-crystal and polycrystalline Fe, which are studied in this work, are of great significance in guiding the mechanistic study of the effects of doping and defects on the strength of Fe-based materials.

Enhancing Mechanical Properties: Exploring the Effect of Annealing Temperature on Wire Arc Additively Manufactured High-Strength Steel.

This study investigates the mechanical properties of exceptionally high-strength steel produced by wire and arc additive manufacturing (WAAM), using the 304 stainless steel wire and the low carbon wire (LCS). The study found that annealing treatment can enhance the steel's mechanical properties. The microstructure in the LCS layer changed from ferrite to bainite and then to a mixture of austenite, pearlite, and bainite with increasing annealing temperature. In contrast, the SS layer retained its martensitic structure, albeit with altered lath sizes. The annealing treatment also improved the orientation of the grains in the steel. The optimal annealing temperature observed for the steel was 900 ℃, which resulted in a maximum tensile strength of 1176 MPa along the Y direction and 1255 MPa along the Z direction. Despite the superior mechanical properties, the LCS layer still exhibited failure during tensile testing due to its lower hardness. The study suggests that annealing treatment can be a useful technique for enhancing the mechanical properties of high-strength steel in WAAM applications.

Experimental study on post-fire mechanical properties and fracture behavior of Q690 steel

Fire is one of the most dangerous disasters for high-strength steel structures. After a fire event, the deterioration of mechanical properties of high-strength steel together with the complex stress state generated by external loading may lead to an unexpected failure or collapse of high-strength steel structures. Therefore, it is very critical to evaluate the post-fire behavior of high-strength steel considering stress state effects. This study conducted a series of tests on the Q690 steel specimens after being exposed to high temperatures ranging from 200 to 1000°C with different cooling methods including air-cooling and water-cooling methods. The tested specimens including smooth round specimens, notched round specimens, and grooved plate specimens were designed to consider the failure mode in axisymmetric stress state and plane strain state. To validate the accuracy of the test results, the mechanical properties such as yield strength, ultimate strength, elastic modulus, and ultimate strain were first compared with other test results reported in the literature. The post-fire ultimate strength under different stress states was obtained directly from the test results. Then, the post-fire equivalent plastic strain at fracture of different specimens was obtained from test data together with the finite element analysis results and analyzed under different stress triaxialities and Lode angle parameters. According to this study, an increase in stress triaxiality can result in an increase in ultimate strength but a decrease in the equivalent plastic strain at fracture for Q690 steel after fire. When the exposed temperature was not higher than 800°C, the post-fire strength of the plane strain specimen (Grooved Plane Specimen) was higher than that of the axisymmetric tension specimen (Notched Round Specimen) under a similar stress triaxiality; however, for their post-fire equivalent plastic strain at fracture, the results were opposite.

Mechanical properties and constitutive model of Q690 steel in the fire-cooling stage

Mechanical properties of high-strength steel during the fire-heating and post-fire stages have been extensively investigated, while those during the fire-cooling stage were hardly studied. In this study, based on test data, a cooling stage influence coefficient (λ), namely the ratio of a mechanical property of steel in the cooling stage over that in the heating stage at identical elevated temperature, was specified to quantify the influence of the cooling process on the mechanical properties of steel. It was observed that when the peak heating temperature before cooling is within a certain value, the cooling stage influence coefficients of Young's modulus and strength were close to 1.0, while they were much less than 1.0 after the peak heating temperature exceeds this value. Besides, a series of formulas obtained by curve fitting of test results were proposed for quickly calculating λ of different mechanical properties for Q690 steel, including Young's modulus, yield strengths, ultimate strength, and ultimate strain. Moreover, a simplified constitutive model of Q690 steel was developed to present the stress-strain relationship of it, which is applicable to the fire-heating, fire-cooling and post-fire stages. The presented work enables more reliable structural analysis of high-strength steel structures throughout the whole heating-cooling process.

Local control of microstructure and mechanical properties of high-strength steel in electric arc-based additive manufacturing

Additive manufacturing offers a significant potential for producing metallic parts with distinctly localised microstructures and mechanical properties, commonly known as functional grading. While functional grading is generally accomplished through compositional variations or in-situ thermo-mechanical treatments, variation of process parameters during additive manufacturing can offer a promising alternative approach. Focusing on the electric arc-based additive manufacturing process, this work focuses on the functional grading of high-strength steel (S690 grade) by adjusting the travel speed and inter-pass temperature. Through a combination of thermal simulations and experimental measurements on single bead-on-plate depositions, it is shown that the microstructure and the mechanical properties of parts can be controlled through the rational adjustment of process parameters. A rectangular block was fabricated to demonstrate functional grading using a constant wire feed rate and varying travel speed. The rectangular block consisted of a low heat input (LHI) region deposited between high heat input (HHI) zones. A graded microstructure was obtained with the HHI zones composed of a mixture of polygonal ferrite, acicular ferrite, and bainite, while the LHI region was primarily composed of martensite. The hardness and profilometry-based indentation plastometry measurements indicated that the LHI region exhibited higher hardness (32%) and strength (50%), but lower uniform elongation (80%), compared to the HHI zones. The present study demonstrates the potential to achieve functional grading by adjusting process parameters in electric arc-based additive manufacturing, providing opportunities for tailor-made properties in parts.

Mechanical properties and constitutive model of Q460 steel during the fire-cooling stage

The mechanical properties of steel under fire-induced elevated temperatures is fundamental and crucial for structural fire-resistance design and analysis. Mechanical properties of high-strength steel (HSS) during the fire-heating and post-fire stages have been extensively investigated, while those during the fire-cooling stage were rarely studied. This study investigated the mechanical properties of HSS Q460 during the fire-cooling stage by steady-state tensile experiments Results show that the mechanical performance of HSS Q460 at the fire-cooling stage is not only related to the test temperature (Tt) but also the peak heating temperature (Tp) before cooling for tests. The strength of HSS Q460 at identical Tt shows an overall download trend with the increase of Tp. The mechanical properties of HSS Q460 at the fire-cooling are similar to those at the fire-heating stage when Tp is not greater than a certain value. This value is 600 °C for Young’s modulus and yield strength f0.2, and 400 °C for yield strength f1.0, f2.0 and ultimate strength. When Tp exceeds certain values, the Young’s modulus and strengths at the fire-cooling stage are less than those at the fire-heating stage. Moreover, the mechanical properties of HSS Q460 were compared with those of HSS Q690, indicating that the degradation rules of mechanical properties of HSSs Q460 and Q690 differ during the fire-cooling stage. Besides, formulas were proposed to calculate the mechanical properties and stress–strain relationship of the HSS Q460 during the fire-cooling stage.

Mechanical properties of Q690E/Q960E high-strength steels at low and ultra-low temperatures: Tests and full-range constitutive models

Mechanical properties of high-strength steels (HSSs) at varying temperatures are fundamental information for the design and numerical analysis of cold-region structures and LNG containers using HSSs. Tensile tests on 14 HSS Q690E and 17 Q960E tensile coupons were firstly carried out at six low temperatures of 20, −30, −60, −80, −120 and −165 °C. The required low-temperature environment for the tensile tests was achieved through an insulation chamber with injecting liquid nitrogen (LN) gas. Their results showed that decreasing the low temperature from 20 to −165 °C slightly affected the elastic modulus of both HSS Q690E and Q960E materials, but significantly increased their yield and ultimate strengths. In addition, ductile failure occurred to all the HSS coupons at low temperatures, and the ductility of HSS was not compromised. Finally, this paper developed full-range constitutive models to describe stress–strain laws of HSS Q690E and Q960E at a low-temperature range of −165 ∼ 20 °C. Validations against the reported test results showed that the proposed full-range constitutive model reasonably estimated the stress–strain behaviours of Q690E and Q960E HSSs at low temperatures.

Microstructure and Mechanical Properties of High-Strength Steel with Improved Machinability

Microstructure and Mechanical Properties of High-Strength Steel with Improved Machinability

Lightweight, package and performance improvements of a shock tower by using steel–aluminium hybrid-casting technique

In this work, a shock tower of a mid-size vehicle using steel (St)–aluminium (Al) hybrid-casting technology was developed with current shock towers as a benchmark. The use of this hybrid-casting technology, which features a ductile material connection between steel and cast aluminium, makes it possible to combine the design advantages of cast aluminium with the mechanical properties of high-strength steels. Based on this combination, a new shock tower concept was developed that offers advantages over the state of the art in terms of package, weight, stiffness and crash performance. To develop the new shock tower, connection points and package spaces in the periphery of the Honda Accord MY 2011 were analysed and defined. Based on quasi-static misuse load cases and topology optimization, it was possible to develop a load-compliant rib structure for the hybrid-cast shock tower reinforced by steel in the dome area. A so-called tension band for the IIHS small overlap crashworthiness evaluation test (SOL) was also integrated into the new shock tower to ensure homogeneous load distribution. The new shock tower was tested virtually in comparison with the reference steel shock tower and an Al-cast shock tower in quasi-static and dynamic crash load cases. In the quasi-static test, the hybrid-cast shock tower showed significantly increased stiffness. In the dynamic load cases, a significant overall homogenization of force distribution on the existing load paths in die front body structure was achieved. In addition, 5 mm package space for spring and damper could be gained for better driving behaviours of the car.

Post-fire performance of high-strength steel plate girders developing post-buckling capacity

Fire is a frequent disaster, which has lasting effects on mechanical properties of high-strength steel (HSS). For HSS structures after fire, it is important to assess if the structure can continue to service, should be repaired or demolished. The experimental and numerical investigations are performed to investigate the effects of exposure temperature on residual resistance of Q690 HSS plate girder considering the post-buckling capacity. In this study, the post-fire mechanical properties of Q690 HSS are investigated experimentally. The appearance characteristics of Q690 HSS with different exposure temperatures are obtained. The effects of exposure temperature on mechanical properties of Q690 HSS were discussed. To investigate the residual resistance of Q690 HSS plate girder after fire, the post-fire mechanical properties of Q690 HSS are introduced in the plate girder model, which has been strictly validated. Based on numerical results, the effect of exposure temperature on the initial stiffness of HSS plate girder can be ignored. The nonlinear properties of the Q690 HSS have the certain effect on the nonlinear characteristics of resistant curve of Q690 HSS plate girder considering the post-buckling capacity, which are influenced by the exposure temperature. The effect of exposure temperature on the buckling resistance of Q690 HSS plate girder is relatively small. With the raise of exposure temperature, the postbuckling capacity of Q690 HSS plate girder decreases. Effects of exposure temperature on the resistant properties of Q690 HSS plate girder were investigated, which were quantified by a suggested equation. The increase of exposure temperature has an adverse effect on the proportion of safety reserve of the Q690 HSS plate girder with post-buckling capacity.

Determination of residual stress evolution during repair welding of high-strength steel components

During the assembly of steel structures, unacceptable weld defects may be found. An economical solution is local thermal gouging of the affected areas and re-welding. Due to high shrinkage restraints of repair weld and surrounding structure, high global and local welding stresses superimpose, and may lead to cracking and component failure, especially in connection with the degraded microstructure and mechanical properties of high-strength steels during the repair process. Component-related investigations of high-strength steels (FOSTA P1311/IGF20162N) focus on welding residual stress evolution during local thermal gouging and rewelding. In this study, repair welding of S500MLO (EN 10225) is carried out using in-situ digital image correlation (DIC) and ex-situ X-ray diffraction (XRD) to analyse strains and stresses. Self-restrained slit specimen geometries were identified representing defined rigidity conditions of repair welds of real components, which were quantified using the restraint intensity concept. The specimens were rewelded with constant welding heat control and parameters. Weld specimens exhibited significantly increased transverse residual stresses with higher transverse restraint intensities, in the weld metal, and in the heat affected zone. Transverse stresses along the weld seam decrease at the weld seam ends leading to different stress state during gouging and welding. XRD analysis of the longitudinal and transverse local residual stresses after cooling to RT showed a good comparability with global DIC analyses.

Mechanical Properties of ASTM A572 Grades 50 and 60 Steels at High Temperatures

Studies involving the mechanical properties of high-strength steel (HSS) at elevated temperatures have received considerable attention in recent years. However, current research on HSS at high temperatures is lacking. As a result, the design of fire-protective steel structures with high standards is not sufficiently conservative or safe. This study investigates the effect that elevated temperatures have on the mechanical properties of ASTM A572 Gr. 50 and 60 steels. Reduction factors for the yield strength, tensile strength, and elastic modulus were derived and compared with the standard (AISC, EN1993-1-2) and previous studies (NIST). This study also provides extensive data on the reduction factors for the yield strength, tensile strength, and elastic modulus of mild steel (MS), HSS, and very-high-strength steel (VHSS). The reduction factor for the yield strength was analyzed by expanding the strain level up to 20%. Equations for the yield strength, tensile strength, and elastic modulus were proposed. In future studies, various strains should be analyzed according to the grade of the steel, with the derivation of a reduction factor that considers the plastic strain of the steel. Hence, the findings reported in this study generated a database that can be applied to fire safety design or performance-based fire-resistant design.

Mechanical properties of TMCP high strength steels with different strength grades at elevated temperatures

High strength steel (HSS) is attracting increasing attention because of its economic and environmental benefits, but special concern should be attached to the fire-resistant design as their mechanical properties may deteriorate faster than normal strength steel (NSS) in a fire. Also, the deterioration may be different when their strength grades are different due to the different heat-treatment processes under which they are produced. Aiming to investigate the different mechanical properties of HSS with different strength grades, a type of S460 steel was tested under axial tension at elevated temperatures using steady-state and transient-state methods, and compared with a type of S690 steel, both are produced in thermo-mechanical controlled process (TMCP) but through different routes. It is discovered that the elastic moduli of S460 steel and S690 steel were similar at elevated temperatures but their strengths deteriorated differently; as their strengths turned to be comparable under fire, the structural members made from them could achieve a similar load-carrying capacity. When compared with NSS structural members with the same conditions in fire, said S460 and S690 steel members reserve no advantages arising from their high strength beyond 400 °C.

The Influence of Hot-Dip Galvanizing on the Mechanical Properties of High-Strength Steels.

Modern high-strength steels achieve their strength exclusively through the manufacturing process, as the chemical composition of these steels is very similar to the composition of standard-quality steels. Typically, hot-dip galvanizing is used to form a protective zinc layer on the steel parts of structures; nonetheless, the material is exposed to high temperatures during the process. With high-strength steels, this can lead to deterioration of the mechanical properties. This study aims to experimentally examine and evaluate the extent of deterioration of the mechanical properties of high-strength-steel members. The effect was studied on specimens made of three different types of steel with the yield strength ranging from 460 to 1250 MPa. For each type of steel, selected mechanical properties—yield strength, tensile strength, and hardness—were determined on specimens with and without hot-dip galvanization, and the obtained results were mutually compared. Our study shows a significant impact of the hot-dip galvanization process on the mechanical properties of some high-strength steels. With the studied types of steel, the yield strength decreased by up to 18%, the tensile strength by up to 13%, and the hardness by up to 55%.

Effects of the Arctic low temperature on mechanical properties of Q690 and Q960 high-strength steels

Mechanical properties of high strength steel (HSS) are fundamental information to analyse and design HSS structures. This study extends the applications of HSS Q690 and Q960 to the Arctic infrastructures. The mechanical properties of HSS Q690/Q960 at the low temperature range of −80~20℃ were studied through performing standard tensile tests on 20 HSS Q690 coupons and 19 Q960 coupons in cooling chamber with LNG, which simulated the Arctic low-temperature environment. The influences of low-temperature levels, thickness of HSS plate, and HSS type on mechanical properties of HSS Q690/Q960 were in-depth investigated. The test results showed that the Arctic low temperature increased the modulus, yield and ultimate strength of HSS Q690/Q960, but did not reduced the ductility of these two types of HSS. In addition, all the tensile coupons of HSS failed in ductile mode without brittle fracture. This paper also developed constitutive models for HSS Q690/Q960 at low temperatures of −80~20℃. Validations showed that the developed constitutive models estimated reasonably well mechanical properties of HSS Q690/Q960 at low temperatures of −80~20℃.