Generation Advanced High Strength Steels Research Articles

Characterisation of Compressive Behaviour of Low-Carbon and Third Generation Advanced High Strength Steel Sheets with Freely Movable Anti-buckling Bars

Measuring the compressive behaviour of sheet materials is an important process for understanding the material behaviour and numerical simulation of metal forming. The application of side force on both surfaces of a specimen in the thickness direction is an effective way to prevent buckling when conducting compressive tests. However, the side effects of side forces (such as the biaxial stress state and non-uniform deformation) make it difficult to interpret the measured data and derive the intrinsic compressive behaviour. It is even more difficult for materials with tension–compression asymmetry such as steels that undergo transformation-induced plasticity. In this study, a novel design for a sheet compression tester was developed with freely movable anti-buckling bars on both sides of the specimen to prevent buckling during in-plane compressive loading. Tensile and compressive tests under side force were conducted for low-carbon steel using the digital image correlation method. The raw tensile and compressive stress–strain data of the low-carbon steel showed apparent flow stress asymmetry of tension and compression, originating from the biaxial and thickness effects. A finite element method-based data correction procedure was suggested and validated for the low-carbon steel. The third generation advanced high strength steels showed intrinsic tension–compression asymmetry at room temperature whereas the asymmetry was significantly reduced at 175 °C.

Microstructure and Mechanical properties of Medium Manganese Steels

Medium manganese steel is a 3rd generation advanced high strength steel, which contains 3–12 wt% manganese. This steel is highly suitable material for the automobile industry due to good balance between the cost of material and the tensile properties. It has got the optimum amount of strength and elongation. Initially the microstructure of this steel after cold rolling is deformed martensite (ά). Intercritical annealing is done to get moderate amount of retained austenite (γr) having good stability, which leads to higher strength and better ductility due to transformation-induced plasticity (TRIP). The formation of stable retained austenite (γr) by intercritical annealing process is our prime interest. Medium Manganese steel usually exhibits austenitic-ferrite microstructure after the intercritical annealing process, which makes the steel suitable for 3rd generation advanced high strength steels as this microstructure gives good amount of elongation. From this study we get best combination of intercritical annealing and quenching to get the required microstructures and properties. Thermo-calc software is used to find the optimum temperature for intercritical annealing at which we get optimum amount of retained austenite (γr) containing maximum manganese. Tensile tests were carried out at different pre-strain level from the samples taken in both rolling and transverse directions. Samples were prepared for SEM and optical microscope characterization to see the changes from initial microstructure of the steel. X-ray diffraction was also done to find out the amount of retained austenite that we are getting from tensile sample after pre-strain it to different level. It was concluded that after intercritical annealing at 650 °C for 2 h the steel sample have approximately 22% retained austenite and ultimate tensile strength (UTS) of 843 MPa and total elongation of around 36%.

Alloying Influences on the Zn-Assisted Liquid Metal Embrittlement (Lme) Susceptibility of 3rd Generation Advanced High Strength Steels

Alloying Influences on the Zn-Assisted Liquid Metal Embrittlement (Lme) Susceptibility of 3rd Generation Advanced High Strength Steels

Local formability of medium-Mn steel

• Local formability of medium-Mn steel is described by forming limits at fracture. • Ductile fracture occurs prior to plastic localization in MMnS. • Large uniform elongation indicates superior global formability of MMnS. • Local formability of MMnS is not enhanced by pronounced strain hardening. Global formability and local formability are critical in different metal forming processes. Edge cracking, controlled by the local formability, is a dominant factor limiting the application of advanced high strength steels (AHSS) in automotive industries. The local formability of a medium-Mn steel (MMnS), a promising candidate of the third generation of AHSS, is evaluated based on forming limit curves at fracture and compared with a dual-phase DP1000 steel using the damage mechanics approach. The superior tensile properties of the investigated MMnS, high tensile strength, pronounced strain hardening, large uniform and total elongation, lead to a very good global formability, which is an indicator of necking resistance. However, the local formability of the investigated MMnS, which is an indicator of fracture resistance and quantified by the plastic strain at fracture under different stress states, is worse than the DP1000 steel. By comparing the local and global formability of the two AHSS, it is confirmed that ductile fracture is the dominant failure mode in the MMnS and the onset of localized necking occurs prior to ductile fracture in the DP1000 steel. To achieve an accurate determination of the local formability, the effects of stress states need to be considered, which cannot be derived explicitly from uniaxial tensile tests. In addition to tensile properties, more attention should be paid to the local formability of new AHSS to assess their potential application in automotive industries.

Impact toughness of an electron-beam welded 0.2C direct-quenched and partitioned steel

Third generation advanced high-strength steels, e.g., quenched and partitioned steels, are forthcoming structural materials, which consist of a martensitic matrix and a substantial proportion of stabilized residual austenite for improved deformability. A novel less energy-intensive processing route of direct-quenching and partitioning advances this concept by facilitating carbon partitioning to untransformed austenite directly from the quench-stop temperature. However, a major challenge also with these steels is how to maintain structural integrity in the welded end-products after additional heat-input reaching above a temperature where given microstructure is still stable. Heat-input limiting beam welding processes are a solution to this by minimizing degradation of the heat-affected zone (HAZ) and producing even-strength welded joints for S1100 and above. In this study, we report toughness properties of an electron-beam (EB) welded 0.2C-1.5Mn-0.5Si-0.8Al-1.1Cr-0.8Ni (wt.%) direct-quenched and partitioned steel (DQ&P) having a yield strength of ∼1100 MPa, and a direct-quenched (DQ) was used as a reference. Low-temperature post-weld heat treatment (PWHT) was considered, too. Weld seam, coarse-grained HAZ, and the base materials were tested for impact toughness. Both the DQ and DQ&P base materials have excellent impact toughness transition temperatures T28J below -100°C. The weld seam has very good low-temperature toughness already at this stage of optimisation with T28J of -66°C, which shows robustness of the chosen alloy. Increased residual austenite content increased upper shelf toughness but not T28J. Furthermore, both the DQ and DQP HAZs have T28J below -70°C, pointing to the weld seam as the weakest link. PWHT reduced low-temperature impact toughness in all the cases with T28J being above -40°C, clearly demanding reassessment of its feasibility.

Effects of CGL-compatible thermal processing, starting microstructure, and Sn micro-alloying on the mechanical properties of a medium-Mn third generation advanced high strength steel

Two prototype medium-Mn third generation advanced high strength steels (Fe-0.2C-6Mn-1.5Si-0.5Al-0.5Cr-xSn wt%, x = 0 or 0.05) were annealed with continuous galvanizing line (CGL) compatible thermal processing parameters. It was found that the starting microstructure, intercritical annealing (IA) parameters and Sn micro-alloying had a significant effect on the volume fraction, stability, and deformation-induced transformation kinetics of the retained austenite and, therefore, the resultant mechanical properties of the steels. It was shown that Sn segregation at the carbide/matrix interface inhibited C diffusion and consequently, reduced carbide dissolution and precipitation kinetics during intercritical annealing. This resulted in lower volume fraction of retained austenite with lower stability after intercritical annealing of the as-received cold-rolled Sn-containing alloy. CGL-compatible IA treatments, therefore, were only able to produce target 3G-AHSS mechanical properties in the as-received cold-rolled alloy without Sn micro-alloying. However, the martensitic starting microstructures were more robust in achieving the 3G-AHSS target mechanical properties for both alloys. A martensitic starting microstructure resulted in higher volume fraction of stable retained austenite for 650 °C and 675 °C IA treatments. Gradual transformation of this stable retained austenite to martensite (TRIP effect) and mechanical twinning (TWIP effect) resulted in high sustained work hardening rates during plastic deformation. As a result, a favourable strength/ductility balance was achieved in the prototype steels. Moreover, Sn micro-alloying did not have a significant detrimental effect on the mechanical properties for the martensitic starting microstructure heat treatments which achieved target 3G-AHSS properties. This is promising in terms of developing a CGL-compatible processing route for these prototype steels.

Early stages of liquid-metal embrittlement in an advanced high-strength steel

Grain-boundary degradation via liquid-metal embrittlement (LME) is a prominent and long-standing failure process in next generation advanced high-strength steels. Here we reveal, well ahead of the crack tip, the presences of nano-scale grains of intermetallic phases in Zn-infiltrated but uncracked grain boundaries with scanning- and 4D transmission electron microscopy. Instead of the often-reported Zn-rich Fe-Zn intermetallics, the nano-scale phase in the uncracked infiltrated grain boundaries is identified as the Γ-phase, and its presence reveals the local enhancement of strain heterogeneities in the grain boundary network. Based on these observations, we argue that intermetallic phase formation is not occurring after cracking and subsequent liquid Zn infiltration but is instead one of the primary nanoscopic drivers for grain-boundary weakening and crack initiation. These findings shift the focus of LME from micro- and meso-scale crack investigations to the very early stages immediately following Zn diffusion, after which secondary phase nucleation and growth emerge as the root-cause for failure.

Impact of liquid metal embrittlement inner cracks on the mechanical behavior of 3rd generation advanced high strength steel spot welds

Controlled degradation of the welding parameters is used in this work to produce homogeneous advanced high strength steel (AHSS) resistance spot welds with significant liquid metal embrittlement (LME) inner cracks and study their impact on mechanical behavior. For the first time, the effect of LME inner cracks on damage development is monitored using interrupted cross tension and tensile shear test. Despite their potential effect on the stress state at the notch tip, the observed LME inner cracks seldom contribute to the final fracture path of the welds, leaving the load bearing capacity and failure type of the welds mostly unaffected in tensile shear or even increased in cross tension.

A Comparative Evaluation of Third-Generation Advanced High-Strength Steels for Automotive Forming and Crash Applications.

While the third generation of advanced high-strength steels (3rd Gen AHSS) have increasingly gained attention for automotive lightweighting, it remains unclear to what extent the developed methodologies for the conventional dual-phase (DP) steels are applicable to this new class of steels. The present paper provides a comprehensive study on the constitutive, formability, tribology, and fracture behavior of three commercial 3rd Gen AHSS with an ultimate strength level ranging from 980 to 1180 MPa which are contrasted with two DP steels of the same strength levels and the 590R AHSS. The hardening response to large strain levels was determined experimentally using tensile and shear tests and then evaluated in 3D simulations of tensile tests. In general, the strain rate sensitivity of the two 3rd Gen 1180 AHSS was significantly different as one grade exhibited larger transformation-induced behavior. The in-plane formability of the three 1180 MPa steels was similar but with a stark contrast in the local formability whereas the opposite trend was observed for the 3rd Gen 980 and the DP980 steel. The forming limit curves could be accurately predicted using the experimentally measured hardening behavior and the deterministic modified Bressan–Williams through-thickness shear model or the linearized Modified Maximum Force Criterion. The resistance to sliding of the three 3rd Gen AHSS in the Twist Compression Test revealed a comparable coefficient of friction to the 590R except for the electro-galvanized 3rd Gen 1180 V1. An efficient experimental approach to fracture characterization for AHSS was developed that exploits tool contact and bending to obtain fracture strains on the surface of the specimen by suppressing necking. Miniature conical hole expansion, biaxial punch tests, and the VDA 238-100 bend test were performed to construct stress-state dependent fracture loci for use in forming and crash simulations. It is demonstrated that, the 3rd Gen 1180 V2 can potentially replace the DP980 steel in terms of both the global and local formability.

Analysis of precipitation characteristics of TiC at different quenching and partitioning temperatures and its effect on the mechanical properties

Intercritical annealing, quenching and partitioning (I&QP) steel is the most promising one of the third generation advanced high-strength steel. However, iron carbides are inevitable in I&QP steel. They can consume the carbon partitioning from supersaturated martensite to austenite and thus reduce the stability of austenite. Furthermore, the transition iron carbides, i.e., ε and η, are harmful to the toughness and ductility. In this work, the addition of Ti has been proved to effectively suppress the formation of the transition iron carbides. The precipitation characteristics of TiC at different quenching and partitioning temperatures (QT and PT) was investigated mainly by transmission electron microscopy, small angle neutron scattering and three-dimensional atom probe tomography techniques. The results showed that more carbides precipitated at the PT of 450 °C compared with that of 400 °C, and the volume fraction of TiC decreased with the increase of QT at the PT of 450 °C, which are mainly determined by the precipitation kinetics and initial martensite fraction. The nano-sized TiC particles are formed at the dislocations within the martensite and austenite and the interface between the two phases, indicating that the high dislocation density can promote the precipitation of TiC. However, the precipitation of TiC decreases the carbon content of retained austenite and thus affects its stability; moreover, the formation of TiC consumes the carbon atoms pinning the dislocations and thus has an important effect on the yield strength and yield platform. This work is essential for suppressing the formation of transition iron carbides, understanding the precipitation characteristics of TiC and designing the I&QP steel with improved mechanical properties.

Study of Diffusible Behavior of Hydrogen in First Generation Advanced High Strength Steels

This study deals with microstructural influence on the H permeation behavior of Dual-Phase (DP) and Complex Phase (CP) steels using electrochemical permeation studies. The H diffusion coefficients in DP steels (DP800: 1.65 × 10−10 m2·s−1, DP1000: 1.58 × 10−10 m2·s−1) are half of that found in CP steels (3.07 × 10−10 m2·s−1).The banded microstructure along the specimen thickness and higher C content of the DP led to high H diffusivity of DP steels. The lower total H concentration along with a higher fraction of H was present in the stronger traps in CP steels suggest a better HE resistance of this steel. The H distribution in the specimens was non-uniform, with a higher H concentration speculated near the charging surface.

New Ms-formula for exact microstructural prediction of modern 3rd generation AHSS chemistries

The exact adjustment of the individual microstructural constituents, in particular the optimum volume fraction of metastable retained austenite (RA), is imperative in case of the 3rd generation advanced high strength steels (AHSS). Moreover, it is of vital importance to avoid the formation of fresh martensite (α’) upon final cooling, otherwise the resulting mechanical properties of these novel steel grades will be pronouncedly deteriorated. Therefore, an accurate estimation of the martensite start (Ms) temperature is essential for modelling and development of this steel group. In this context, the present contribution proposes a new relation for the Ms temperature prediction with a remarkably wide composition range based on the 3rd generation AHSS chemistries. Moreover, the new Ms formula allows for an accurate determination of the Ms temperature for both, extremely low and very high C contents, which is not the case in any Ms formula known from literature.

Mechanical properties and failure behavior of resistance spot welded third-generation advanced high strength steels

Abstract Third generation advanced high strength steels are candidates for application in lightweight automotive components due to their remarkable combination of high strength and high ductility. Adoption of these steels is accompanied by challenges to better understand the mechanical performance of their resistance spot welds. In this work, the resistance spot weldability of two third-generation advanced high strength steels, designated herein as 3G-980 and 3G-1180, was investigated. The weldability windows of the steels were developed to determine process robustness and acceptable welding zones for different welding times. The mechanical properties of the spot welds were evaluated by conducting lap-shear and cross tension tests for different nugget sizes. The investigated steels exhibited both interfacial and pullout/partial-pullout failure modes during mechanical testing and the transition points between the failure modes were identified as a function of the welding condition. The optimal weld strengths of the 3G-980 and 3G-1180 samples were 28.7 kN and 30.7 kN for lap-shear and 10.2 kN and 9.4 kN for cross tension specimens respectively. Interestingly, it was shown that even interfacial failure of the joints can demonstrate load-bearing capacities close to the optimized condition in lap-shear testing. Also, in contrast to 3G-980, microhardness measurement of different weldment regions revealed a local drop in the hardness of the sub-critical heat-affected zone for 3G-1180. These measurements were correlated to the crack initiation region and propagation paths of interrupted lap-shear specimens for both steels.

Constitutive, Formability, and Fracture Characterization of 3rd Gen AHSS with an Ultimate Tensile Strength of 1180 MPa

<div class="section abstract"><div class="htmlview paragraph">The superior formability and local ductility of the emerging class of third generation of advanced high-strength steels (3rd Gen AHSS) compared to their conventional counterparts of the same strength level offer significant advantages for automotive lightweighting and enhanced crash performance. Nevertheless, studies on the material behavior of 3rd Gen AHSS have been limited and there is some uncertainty surrounding the applicability of developed methodologies for conventional dual-phase (DP) steels to this new class of AHSS. The present paper provides a comprehensive study on the quasi-static and dynamic constitutive behavior, formability characterization and prediction, and the fracture behavior of two commercial 3rd Gen AHSS with an ultimate strength of 1180 MPa that will be contrasted with a conventional DP1180. The hardening response to large strain levels was determined experimentally using tensile and shear tests and then validated with 3-D simulations of tensile tests. In general, the strain rate sensitivity of the two 3rd Gen AHSS was significantly different as one grade exhibited larger transformation-induced behavior. The in-plane formability of the three 1180 MPa steels determined using Marciniak tests was similar but with a stark contrast in the local formability for the 3rd Gen AHSS. The forming limit curves could be accurately predicted using the experimentally measured hardening behaviour and the modified Bressan-Williams through-thickness shear model. An efficient experimental approach to fracture characterization for AHSS was developed that exploits tool contact and bending to obtain fracture strains on the surface of the specimen by suppressing necking. Miniature conical hole expansion and biaxial punch tests are used along with the VDA 238-100 bend test.</div></div>

Development of Advanced High Strength Automotive Steels

Abstract In recent decades, the automotive industry has faced ever-increasing demands. Increasing requirements can be observed in terms of both consumer expectations and legal requirements. On the consumer side, there is a demand for cars that are as economical as possible with lower fuel consumption, but providing also greater comfort and safety. These requirements are accompanied, from a legal point of view by more rigorous environmental regulations and requirements concerning the reduction of harmful emissions. Meeting these often-contradictory requirements is a growing challenge for car manufacturers and raw material suppliers, as well. Meeting the requirements in the most versatile way has resulted in tremendous progress over the last 40–50 years, both in the automotive industry and in the production and development of raw materials. The first part of this series of papers summarizes the main requirements in the automotive industry, as the main driving forces for material developments. Furthermore, the main types and properties of traditional high-strength steels, as well as the so-called first-generation Advanced High-Strength Steels will be introduced. In the second part, the main types and manufacturing processes of second generation advanced high-strength steels will be analyzed and some of the current steel developments will be presented through the results of the three generations of Advanced High-Strength Steels.

Enhanced Cross-Tension Property of the Resistance Spot Welded Medium-Mn Steel by In Situ Microstructure Tailoring

Medium Mn steels, one of the most promising 3rd generation advanced high strength steels (AHSS), achieve an encouraging trade-off between the outstanding mechanical property and the production cost. As a typical medium Mn steel, 7Mn steels have superior mechanical property but their poor cross-tension property becomes the Achilles' heel, hindering the application in the automotive industry. The current study focuses on the cross-tension property of the resistance spot weld of 7Mn steel. Generally, martensite is produced in the nugget during the resistance spot welding (RSW). However, the microstructure in the weld nugget can be correspondingly tuned by directly optimizing the welding parameters. With post-weld pulses, in situ tempering occurs, which can decrease the segregation of Mn existing along martensite lath boundaries and facilitate the microstructure transition from martensite to tempered martensite. The tuning on the nugget microstructure facilitates the increase of cross-tension strength (CTS) from 1.5 to 3.7 kN. Although both cases fail in an interfacial fracture mode, a partial ductile fracture is demonstrated in the specimen with post-weld treatment, which is attributed to the occurrence of low-carbon α phase and second phase particles. This study elucidates that the decrease of segregation and the microstructure transition in the nugget are the dominant factor determining the CTS. It is therefore demonstrated that the reduction of Mn segregation and the formation of tempered martensite can increase the weldability of RSW joints of the medium Mn steels.

Prediction and evaluation of optimum quenching temperature and microstructure in a 1300 MPa ultra-high-strength Q&P steel

The quenching and partitioning steel is the representative of the third generation of advanced high-strength steel. The effect of quenching temperature on the microstructure and mechanical property of ferrite-containing quenching and partitioning steel was studied by intercritical annealing quenching and partitioning processes. When preparing a test steel with a tensile strength of 1300 MPa and total elongation of 19%, it is found that the actual optimum quenching temperature was lower than that calculated according to the constrained carbon equilibrium. The results indicate that the martensite start temperature of the austenite was overestimated when considering the diffusion of carbon only. Austenite grain size which is affected by low temperature and the existence of ferrite during intercritical annealing influenced the optimum quenching temperature. A scheme considering the diffusion of various alloying elements and austenite grain size was proposed and verified. Using this scheme, the optimum quenching temperature of intercritically annealed quenching and partitioning steel with complex microstructures was well predicted.

Corrosion Mechanisms of High-Mn Twinning-Induced Plasticity (TWIP) Steels: A Critical Review

Twinning-induced plasticity (TWIP) steels have higher strength and ductility than conventional steels. Deformation mechanisms producing twins that prevent gliding and stacking of dislocations cause a higher ductility than that of steel grades with the same strength. TWIP steels are considered to be within the new generation of advanced high-strength steels (AHSS). However, some aspects, such as the corrosion resistance and performance in service of TWIP steel materials, need more research. Application of TWIP steels in the automotive industry requires a proper investigation of corrosion behavior and corrosion mechanisms, which would indicate the optimum degree of protection and the possible decrease in costs. In general, Fe−Mn-based TWIP steel alloys can passivate in oxidizing acid, neutral, and basic solutions, however they cannot passivate in reducing acid or active chloride solutions. TWIP steels have become as a potential material of interest for automotive applications due to their effectiveness, impact resistance, and negligible harm to the environment. The mechanical and corrosion performance of TWIP steels is subjected to the manufacturing and processing steps, like forging and casting, elemental composition, and thermo-mechanical treatment. Corrosion of TWIP steels caused by both intrinsic and extrinsic factors has posed a serious problem for their use. Passivity breakdown caused by pitting, and galvanic corrosion due to phase segregation are widely described and their critical mechanisms examined. Numerous studies have been performed to study corrosion behavior and passivation of TWIP steel. Despite the large number of articles on corrosion, few comprehensive reports have been published on this topic. The current trend for development of corrosion resistance TWIP steel is thoroughly studied and represented, showing the key mechanisms and factors influencing corrosion processes, and its consequences on TWIP steel. In addition, suggestions for future works and gaps in the literature are considered.

Deformation behavior of Austenitic stainless steel at subzero temperature

Austenitic stainless steel is one of the second generation advanced high strength steel which finds application in automobile, aerospace and cryogenic components. The component made of austenitic steel might operate in subzero temperature condition because of its excellent formability even at subzero temperature. In the present work several tensile tests were performed on austenitic stainless-steel sheet of thickness 1.2 mm at 0°C, -40°C, -80°C, -120°C and at different strain rates of 0.01/sec,0.001/sec,0.0001/sec. The resultant mechanical properties, like yield strength, tensile strength, elongation percent and strain hardening exponent, along with phase fractions and microstructural properties were analyzed to understand the reasons for change in mechanical properties, on comparing with room temperature properties. It was noticed that tensile strength is 635 Mpa, & strain hardening exponent is 0.38 at room temperature (25 °C) and tensile strength is 1236 Mpa, & strain hardening exponent is O.49 at -120°C. Similarly, XRD characterization revealed that strain induced martensite increased from zero percent at 25°C (room temperature) to 57 percent at-120°C Similarly EBSD characterization revealed that grain average misorientation which also increased from room temperature to-120°C.

The effect of Nb on the high strain rate hydrogen embrittlement of Q&P steel

Quenching and Partitioning (Q&P) steels are, due to their excellent combination of strength and ductility, seen as good candidates for the third generation advanced high strength steels (AHSS). Although the TRIP effect is beneficial for the overall mechanical behaviour of these steels it potentially can have detrimental effects when strained in a hydrogenenriched environment. The solubility of hydrogen is high in austenite but low in high carbon martensite. Martensite is even in the absence of hydrogen already a possible damage initiation spot. The effect of hydrogen under static and dynamic tensile loading was evaluated in a Q&P and a Nb micro-alloyed Q&P steel. Experiments were carried out under a strain rate ranging from 0.03 s-1 till 500 s-1 and correlated with the hydrogen uptake characterised via thermal desorption spectroscopy (TDS). The presence of Nb resulted in a 25% increase in the hydrogen uptake capacity. A higher susceptibility to hydrogen was observed in the Nb steel partially due to the high hydrogen fraction, but also because of the larger fraction of low stability austenite. However, when tested under dynamic conditions the hydrogen susceptibility is minor and even improved in the micro-alloyed Q&P steel compared to the standard Q&P steel.