Advanced High Strength Steels Research Articles

Experimental characterization of forming behavior of 3rd GEN AHSS

Third generation of Advanced High Strength Steels (3rd GEN AHSS) for cold stamping combine superior formability and ductility at higher strengths. Their combination with the Laser Welded Blanks (LWB) technology is one answer to improve safety and weight reduction of vehicules for automakers. In this study, a Nakazima test is used to evaluate the forming behavior of LWB based 3rd GEN AHSS and to estimate their Forming Limit. Tests equipped with a Digital Image Correlation (DIC) system are carried out for several materials combinations in order to understand the influence of the weld on LWB formability. The experimental results are used to determine the critical area during this forming test and a dedicated FLC for each material combination. The location of the onset of failure is investigated, from optical observations, DIC data as well as modeling. It appears that different failure locations are obtained depending on both AHSS combination and strain paths. In order to improve the accuracy of the determination of the onset of the failure, a method is proposed to evaluate the failure risk in base metal and in the vicinity of the weld. The improvement of failure prediction will be very helpful to analyze part formability and provide design recommendations.

A new anisotropic-asymmetric yield criterion covering wider stress states in sheet metal forming

The stress state-dependent plastic anisotropy of sheet metals has drawn significant attention. Nevertheless, existing phenomenological plasticity models limitedly capture the distinctive yield strengths and plastic flow (r-values) under a wide range of stress states covering uniaxial tension (UT), equi-biaxial tension (EBT), uniaxial compression (UC), plane strain tension (PS), and pure shear (SH). This study aims to propose a new anisotropic-asymmetric plasticity model with enhanced flexibility based on the non-associated flow rule. The yield function is formulated based on the additive coupling of two stress potentials: coupling between a new anisotropic pressure-sensitive fourth-order polynomial function, and an isotropic stress invariant-based function. Then, all the anisotropic/asymmetric parameters of the yield function can be analytically identified from 7 yield stresses measured from UT tests along 0°, 45°, and 90° to the rolling direction (RD), UC tests along 0° and 90° to the RD, EBT test, and SH test along 45° to the RD. Within the scheme of no-associated flow rule, a new asymmetric fourth-order plastic potential function is proposed to analytically describe the plastic flow of sheet metals under UT, UC, EBT and SH. The additively coupled stress invariant-based term is introduced to adjust the yield stresses and plastic flow under biaxial tension including PS, which leads to an independent description of the yield stresses and plastic strain directions under SH and PS states. Various automotive sheet metals such as advanced high-strength steels, aluminum alloy, and magnesium alloy with strong asymmetry and anisotropy are investigated to verify the proposed yield criterion. The results demonstrate remarkable flexibility and accuracy of the proposed yield criterion against other existing models. Evolving yield surface and plastic potential of a quenching and partitioning steel are accurately captured by the proposed anisotropic-asymmetric yield criterion. Besides, the advantages of new model are further discussed in terms of the proof of yield surface convexity, identified parameters, and the regulation between anisotropy and isotropy in comparison with existing multiplicative coupling method.

On the influence of tool contact stress on tensile instabilities in plane strain stretching and bending of sheet metals

It is well known that the widely used forming limit curve (FLC) based upon in-plane stretching fails to account for the delay in plastic instability in the presence of appreciable bending and tool contact pressure. The recently proposed General Incremental Stability Criterion (GISC) – a generalization of the Modified Maximum Force Criterion (MMFC) to triaxial loading – revealed the dependence of the boundary conditions on the formation of tensile instabilities. Nevertheless, it remains unclear how the combined effect of radial bending stresses and tool contact pressure affects the formation of an acute neck. It is shown in this study that a compressive normal stress causes a shift of the strain state from initial plane strain tension to positive minor strains for material layers within the cross-section. The convex (outer) layer remains in a state of plane strain-plane stress. Accounting for the shift in the strain state is key to capture the dependence of the contact pressure on necking. A simplified modelling approach, which enforces plane strain tension of the cross-section, was able to capture the formability gain due to bending but failed to account for the delay in plastic instability due to the contact pressure. An incremental multi-layer model was required to account for non-monotonic loading due to bending over the punch and the biaxial shift in the strain path caused by the contact pressure. The models were then applied to the experimental forming limits of a 3rd Gen 1180 advanced high strength steel to demonstrate the formability gains under triaxial loading due to the contact pressure.

Heat treatment effects on microstructure and properties of advanced high-strength steels

The effects of the austempering time on the microstructure and properties of two third-generation high-strength steels are investigated for austempering (TBF), and for quenching and partitioning (QP). The process parameters are chosen according to the capacity of an existing continuous annealing or galvanising line. More than 10% retained austenite can be stabilised although carbides start to form at short overageing times with the simulated cycles. The product of the tensile strength and total elongation for TBF is higher than for QP, and it peaks with increasing austempering time for TBF, but decreases monotonously for QP. The hole expansion capacity for QP is higher than for TBF and it peaks with austempering time for both TBF and QP.

Integrated experimental–numerical investigation of strain partitioning and damage initiation in a low-carbon lath martensitic steel

Lath martensite is a key constituent of advanced high strength steels (AHSS). Its deformation behavior is intrinsically related to its hierarchical structure composed of packets, blocks and laths exhibiting specific topological and orientation relationships. To improve the ductility of lath martensite, it is therefore important not only to clarify the deformation mechanisms but to also gain a better understanding of the damage initiation mechanisms and their connections with local micro-mechanical features. To this end, an integrated experimental–numerical investigation of a low-carbon lath martensitic steel combining large field-of-view high-resolution digital image correlation (HR-DIC) analyses and crystal plasticity (CP) simulations with different modeling assumptions is proposed. The global and local plastic strain field distributions are first comparatively and quantitatively analyzed to clarify the predictive capabilities and limitations of the CP models within the continuum mechanics approximation. The results confirm that plasticity is predominantly carried out by interface sliding so that a model discriminating bulk and boundary elements with respect to their initial in-lath and out-of-lath slip strengths can reasonably reproduce the plastic deformation in terms of spatial distribution and intensity. Severe strain localization is observed along block boundaries with high in-lath Schmid factor, at packet interfaces but also at prior austenite grain boundaries and probably at an annealing twin boundary. The relationship between the strain localization sites and local micro-mechanical fields, in particular the stress triaxiality is discussed.

Springback Control in Complex Sheet-Metal Forming Based on Advanced High-Strength Steel

Sheet-metal forming is one of the most important manufacturing processes in the automotive industry. This study proposes a multiobjective optimisation scheme that controls both sheet-metal formability and springback. First, the mechanical properties of DP590 steel were characterised to obtain the mechanical parameters and forming limit diagram (FLD) of DP590. Then, the FLD, thinning rate, and average springback were selected as forming quality evaluation indices. Response surface tests were then conducted for different process parameters for the A-pillar side-stiffener drawing process to analyse the DP590 steel’s formability and springback. The optimal process parameters for the drawing process were obtained using a multiobjective optimisation algorithm based on an improved particle swarm method. Finally, a springback compensation scheme was proposed based on the results of the multiprocess simulation. The scheme was applied experimentally to the A-pillar side-stiffener drawing process, and the formability and springback compensation performances verified that the scheme successfully and efficiently eliminated springback and rupture in formed DP590 steel.

Influence of the hydraulic press system on advanced high-strength steel formability

Influence of the hydraulic press system on advanced high-strength steel formability

Effects of Diffusible Hydrogen on Fatigue Life of Spot Welds in High-tensile-strength Steel Sheets

Spot-welded advanced high-strength steel (AHSS) sheets are used to decrease car body weight and enhance crashworthiness simultaneously. Such welds are susceptible to hydrogen embrittlement, but research in this area is limited. We have evaluated the hydrogen embrittlement properties of spot welds fabricated with AHSS sheets and clarified the dependence of diffusible hydrogen content and tensile rate on tensile shear strength. However, the performance of spot welds must be evaluated not only by static testing such as tensile shear tests but also by dynamic fatigue testing. Herein, we investigated the effects of diffusible hydrogen on their fatigue life via tensile–shear fatigue tests under cathodic hydrogen charging. The number of cycles to failure and the endurance limit decreased with diffusible hydrogen content. The number of cycles to failure decreased as the constant amplitude-loading frequency declined. The intergranular cracking is more likely to occurs as the amount of diffusible hydrogen increases and as the frequency decreases. These results are explained by hydrogen accumulation and crack growth behavior caused by mechanism for hydrogen-enhanced local plasticity and hydrogen enhancement of the strain-induced generation of vacancies mechanism. Furthermore, the correlation between static testing and dynamic fatigue testing was investigated, and the relationship between fatigue strength and the amount of diffusible hydrogen was in good agreement with the diffusible hydrogen dependence of tensile shear strength obtained from slow rate tensile shear tests under hydrogen charging.

Hydrogen Embrittlement in Martensitic Stainless Steels and Dual Phase Steels

Hydrogen embrittlement is a phenomenon of hydrogen diffusion into a metal alloy that can cause it to become brittle and fracture. Embrittlement is a critical issue in advanced high strength steels, such as martensitic stainless steels (MSS) and dual phase steels (DPS), and hence, it is necessary to understand this phenomenon to avoid premature material failure under similar conditions. The goal of the current research is to study the changes in the mechanical properties of these steels following the introduction of hydrogen. Samples in this study were cut and polished to conduct hardness measurements before and after electrochemical hydrogen charging. The samples were also chemically etched to characterize the microstructure via optical microscopy and stereology. By conduction stereology on the DPS and MSS samples, the sizes of the various phases were able to be recorded. It was observed that an hour of hydrogen charging increased the hardness of DPS sample from 316 ± 12 HV to 384 ± 21 HV. It did not have significant effect on the MSS sample. Ultimately, this research correlated with the known effects of hydrogen presence with the hardness of MSS and DPS.

A review of the processing, microstructure and property relationships in medium Mn steels

ABSTRACT Medium Mn steels are an emerging class of 3rd generation advanced high-strength steels. These steels have received significant attention due to their high strengths, large ductilities and also lower cost compared to their predecessor high Mn Twinning Induced Plasticity (TWIP) steels. Additionally, medium Mn steels have been found to exhibit TWIP and/or Transformation Induced Plasticity (TRIP) effects which can be harnessed to give a high strain hardening rate. Many thermomechanical processing concepts in the literature have been developed, producing multiple microstructure types with differentmechanical properties. The present review therefore aims to summarise the current knowledge of medium Mn steel alloy design especially on the processing, microstructure and property relationships in medium Mn steels. It complements the review of Sun et al. [Physical metallurgy of medium-Mn advanced high-strength steels, Int Mater Rev. 2023.], written independently and in parallel, which focusses more on the phase interfaces and thermodynamics.

Effect of strength of steel sheets on peel tensile strength and failure mode of dissimilar joint of spot welds

Advanced high strength steel sheet (AHSS) is widely used in the automotive body for weight reduction and the improvement of crash performance. Peel tensile strength of spot welds tends to decrease as the steel sheet strength increases. Therefore, in order to further expand the application of AHSS, technology to prevent fracture of spot welds is required. In order to achieve that, it is necessary to understand the characteristics of spot-welded joints and clarify the controlling factors. However, most of the conventional joint strength evaluations have been conducted on the same kind material joints, and there are few systematic studies on the joint strength and fracture mode of dissimilar material joints that are often used in actual vehicles. Therefore, the purpose of this study is to obtain a guideline for grasping the characteristics of dissimilar joints in case of the plug failure. Using various joint shapes, we compared the peel tensile strength and failure position of the same kind material joints and the dissimilar material joints with the TS1500 MPa class hot-stamped steel sheet. As a result, it was found that the deformation behaviour of the test piece affects the joint strength and fracture position of the dissimilar material joints. Specifically, in dissimilar material joints, cross tension joints and double cup-shape tension joints tended to break on the TS1500 MPa class hot-stamped steel side. On the other hand, L-shape tension joints broke on the lower strength material side. From these results, it is considered important to understand their deformation behaviour in order to analyse and predict the fracture phenomenon of spot welds in actual vehicles and automotive parts.

Effect of strength of steel sheets on tensile shear strength and failure mode of dissimilar joint of spot welds

Advanced high strength steel sheet (AHSS) is widely used in the automotive body for weight reduction and the improvement of crash performance. For example, TS1500 MPa class hot stamped steel (1500HS steel) sheet is applied to many vehicle models. However, there is a concern that performance of spot welded joints will decline as steel strength increases. In order to further expand the application of AHSS, it is very important to have knowledge about the characteristics of spot welded joints. In this report, we investigated the tensile shear strength (TSS) of the 1500HS steel sheet. Then, we compared the joint strength and failure position of the same kind material joints and the dissimilar material joints with the 1500HS steel sheet in case of the plug failure. As a result, it was found that TSS of dissimilar material joints depend on base metal strength on lower strength steel side. However, fracture occurred on the 1500HS steel side except for joints with large difference of base metal strength. The mechanism is thought that the effect of strain concentration at the edge of nugget on the 1500HS steel side due to the increase of rotational deformation around the nugget.

Two design strategies for enhancing the thermal stability of bainitic structures

In response to the growing interest of the industry in advanced high-strength steels subjected to extreme operating conditions, two novel grades BainTS and BainNiAlCu bainitic steels have been developed. Since the in-use properties of bainitic steel are directly related to the thermal stability of the bainitic structure, two different alloy design concepts to improve tempering resistance considering various strengthening mechanisms were proposed. BainTS steel follows the standard assumptions of nanocrystalline bainitic steel design, where the retained austenite is stabilized with silicon. A potential secondary hardening was also considered using Cr, Mo, and V alloying additives. On the other hand, BainNiAlCu steel was designed taking into account the precipitation strengthening with intermetallic compounds (nickel aluminide and copper particles). It was revealed that both steels after heat treatment consist of bainitic ferrite laths and a significant content of retained austenite (above 25%) with film and blocky morphologies. After the tempering process (350 °C–650 °C), there was significant structural evolution following the gradual decomposition of the metastable bainitic structure. It was proved that BainTS steel is characterized by the higher thermal stability of both bainitic ferrite and retained austenite compared to BainNiAlCu. Despite this, the hardening effect after tempering BainNiAlCu steel was higher (by about 140 HV compared to heat treatment, approx. 30% increase). The presented results suggest that bainitic steels strengthened with intermetallic phases are extremely promising in the context of future industrialization in applications where operating conditions are exposed to elevated temperatures.

Role of silicon on formation and growth of intermetallic phases during rapid Fe–Zn alloying reaction

The study of liquid metal embrittlement in Fe–Zn systems is challenging because of the high temperature and vapor pressure of Zn, which hinders in-situ investigations with sufficiently high spatial resolution. This is typically associated with subsecond processing steps and the coexistence of a solid substrate and a liquid Zn phase, which renders direct observations at the microstructural scale difficult. In this study, we comprehensively investigate the reactions occurring during the rapid heating and cooling stages of Fe–Zn systems using synchrotron X-ray diffraction. The phase transformation is analyzed for specimens with different interfacial structures, with focus on changes in the coating microstructure above and below the peritectic temperature (782 °C) of the Fe–Zn system. Advanced high-strength steel (AHSS) variants with 1.5 wt% Si content show a prominent destabilizing effect at the onset of Fe–Zn intermetallic compound formation and a simultaneous deceleration of the liquid Zn depletion rate compared with other AHSS alloys containing 0 wt% Si. Furthermore, the addition of Si suppresses the formation of the Γ phase, which is due to turbulence in the outburst Zn at temperatures above 773 K. Consequently, the fraction of Γ phase compounds with accumulated Zn deceases as the exposure time of the liquid Zn to the ferritic matrix increases owing to the solute redistribution of Si in the liquid Zn during rapid heating. Meanwhile, the absence of silicon in the substrate causes the formation of a ζ phase with a low melting point, which delays the formation of liquid Zn via an increase in the melting point of the Zn layer in the early stage of heating. Additionally, the amount of residual liquid Zn decreases due to the rapid depletion of Zn during the liquid initiation stage via an equilibrium Fe/Zn binary phase transformation. The results of this study provide a deeper understanding of the Fe–Zn intermetallic phase transformation and the sensitivity to liquid metal embrittlement of third-generation AHSSs based on silicon content.

Effect of Zn-coating type on intergranular Cu-penetration in steels during weld-brazing

Weld-brazing has emerged as a promising alternative to fusion welding for joining Zn-coated advanced high strength steels (AHSSs) due to its lower heat input, reduced Zn burn-off, and elimination of defects like porosity and blowholes. Additionally, the weld-brazing process minimizes HAZ softening, a phenomenon that can affect joint strength. While weld-brazing can potentially eliminate the liquid metal embrittlement (LME) caused by the penetration of molten Zn into grain boundaries (GBs), this study found that galvanized (GI) DP600 steels were more susceptible to intergranular penetration of Cu into the substrate during weld-brazing compared to galvannealed (GA) DP600. Although no Zn-related LME was observed during arc-brazing with Si-Bronze filler, the potential for Cu-related LME cracks to form in GI coated steels may affect the mechanical integrity of the joint.

Comparison of RSW technologies on DP steels with modified instrumented Charpy impact test

In recent decades the toughness of welded structures becomes more and more important. This trend is particularly valid for the advanced high-strength steels (AHSS), which have reduced toughness comparing to the low-strength structural steels. Dynamic characteristics of the welded joints of the newly developed high-strength steel sheets required by the automotive industry have been neglected, with welding procedures being optimized on the static joint properties, mainly on the tensile-shear force. Up to the present time, testing of dynamic properties of the spot-welded joints has been performed by increased testing speed during tensile testing. The authors have developed a new dynamic testing method and designed new testing equipment for impact bending, which can give a numeric result to characterize the resistance of spot-welded joints against dynamic load. In this paper, this method will be used to evaluate resistance spot-welded joints made on DP600 and DP800 steels with three different technological parameters, comparing long-time welding, short-time welding, and two-pulse welding. The different parameters cause different weld nugget sizes, failure modes, impact energies, and force–time diagrams. All test results show that bigger weld nugget diameters cause better impact energies and impact forces, but the differences are not perfectly correlated.

Effect of plate thickness on mechanical properties and failure behaviors of resistance spot welded advanced high strength steels

In this work, the mechanical properties and failure behaviors of resistance spot welds (RSWs) were investigated under lap-shear (LS) loads. Different types (B1500HS, 340/590DP, 420/780DP, DC03 and DC07) and thicknesses (1.0–1.8 mm) of body-in-white steel plates were welded as LS samples. The microstructures, weld defects and Vickers hardness values of different RSW regions were measured to explain the failure mechanisms of RSWs. Obvious hardness reduction near the heat affected zones of RSWs can induce stress concentration and crack initiation. Moreover, with the increment of plate thicknesses, the increasing peak loads (FLS) values and RSW fracture mode translation (from pull-out fracture to partial interfacial fracture) were also identified. The LS load displacement curves were divided into three ranges. The remarkable correlation between the FLS values and the tensile strengths (σb) of base metals (BMs) was found in the secondary range. Based on this, a new introduced parameter, standardized shear strength (σSS), can quantified the relationship between the FLS values, plate thicknesses and BM σb values, σSS = FLS / T1 / T2 = f(σb). Finally, a FLS prediction model was provided for the LS samples with different materials and plate thicknesses.

Characterization of Retained Austenite in Advanced High-Strength Steel

The retained austenite (RA) in advanced high-strength steels directly affects their plasticity. It is very important for the accurate characterization of their content and types. This paper prepared three specimens with three different Mn contents (1.0%, 1.4%, and 1.7%) that are used to obtain high-strength steel by ultrafast cooling heat treatment. The volume content and distribution of the RA were analysed by an X-ray Debye ring measurement system, electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). In addition, the mechanical tensile test provided the tensile properties and elongation of three specimens. It was finally concluded that when the content of Mn increased, the island-type and thin film-type RA both increased, which may effectively improve the plasticity of the martensitic steels.

AHSS—Construction Material Used in Smart Cities

With the level of development of the smart city, there are more and more research sub-areas in which the latest material and technological solutions are used, enabling the proper management and functioning of these cities. On the one hand, the introduced materials and technologies are designed to facilitate the functioning of residents both in the urban space and at home; on the other hand, the implemented solutions strive to be consistent with the principles of sustainable development. As shown in this article, reports on new technical and technological solutions and their positive and negative effects are strongly emphasized in publications on the development of smart cities. The most highlighted materials research in the smart city area concerns smart materials and their characteristics and applications. A research gap in this area is in the presentation of material solutions, particularly materials intended for the load-bearing structures of vehicles (electric vehicles, flying vehicles) or infrastructure elements (buildings, shelters, etc.) designed to increase the durability of the structure while reducing its weight. This paper aims to comprehensively present the most important research areas related to the functioning of smart cities in light of previous research, with particular emphasis on new material solutions used for thin-walled load-bearing structures in smart cities made of AHSS (advanced high-strength steel). These solutions are very essential for smart cities because their use allows for the installation of additional devices, sensors, transmitters, antennas, etc., without increasing the total weight of the structure; they reduce the number of raw materials used for production (lighter and durable thin structures), ensure lower energy consumption (e.g., lighter vehicles), and also increase the passive safety of systems or increase their lifting capacity (e.g., the possibility of transporting more people using transports at the same time; the possibility of designing and arranging, e.g., green gardens on buildings; etc.). AHSS-welded joints are usually characterized by too-low strength in the base material or a tendency to crack. Thus, the research problem is producing a light and durable AHSS structure using welding processes. The research presented in this article concerns the possibility of producing welded joints using the Metal Active Gas (MAG) process. The test methods include the assessment of the quality of joints, such as through visual examination (VT); according to the requirements of PN-EN ISO 17638; magnetic particle testing (MT); according to PN-EN ISO 17638; and the assessment of the selected mechanical properties, such as tensile strength tests, bending tests, and fatigue strength checks. These methods enable the selection of the correct joints, without welding defects. The results have a practical implication; advanced production technology for obtaining AHSS joints can be used in the construction of the load-bearing elements of mobile vehicles or parts of point infrastructure (shelters, bus stops). The obtained joint is characterized by adequate strength for the production of the assumed structures. The originality of the manuscript is the presentation of a new, cheaper, and uncomplicated solution for obtaining an AHSS joint with good mechanical properties. The application of the presented solution also contributes to sustainable development (lower fuel and material consumption use by mobile vehicles) and may contribute to increasing the load capacity of mobile vehicles (the possibility of transporting more people).

Characterization and Modeling of Anisotropic Fracture of Advanced High-Strength Steel Sheets

<div class="section abstract"><div class="htmlview paragraph">As an engineering approach of balanced complexity and accuracy, the Generalized Incremental Stress-State dependent damage Model (GISSMO) in LS-DYNA<sup>®</sup> has now been widely adopted by the automotive industry to predict metallic materials’ fracture occurrences in both forming and crashworthiness simulations. Calibration of the nominal GISSMO is typically based on material characterization data along a certain representative material orientation. Nevertheless, many rolled or extruded metallic materials, such as advanced high-strength steel (AHSS) sheets, exhibit accentuated anisotropic fracture behavior, even though, notably, some of these materials show comparatively weak anisotropic plasticity in the meantime. Accordingly, in this work, the deformation and fracture behavior of a selected AHSS grade, Q&P980 steel, was first characterized based on a series of mechanical experiments under simple shear, uniaxial tension, plane strain, and equi-biaxial tension conditions. Then, material models were calibrated based on the plasticity and fracture data. Two fracture models, either stress- or strain-based, were applied to fit the fracture loci of the target material, which then could be directly implemented into the material cards in LS-DYNA<sup>®</sup>. Particularly, to simulate the anisotropic fracture behavior of the target material, an extended GISSMO material card (eGISSMO) was introduced and highlighted in this work. Unlike the nominal GISSMO, the eGISSMO integrated the different anisotropic fracture loci and damage accumulation along three material orientations (longitudinal, diagonal, and transverse) into a single material card. In the subsequent validation based on a customized three-point-bending (3PB) testing setup on hat-section samples, only the finite element (FE) model using the calibrated eGISSMO successfully simulated the anisotropic fracture bifurcation observed in the actual experiments.</div></div>