Third-generation Advanced High-strength Steels Research Articles

Experimental characterization and modeling of complex anisotropic hardening in quenching and partitioning (Q&P) steel subject to biaxial non-proportional loadings

Quenching and partitioning (Q&P) steels, as one of the third-generation advanced high strength steels, exhibit complex anisotropic hardening behavior under biaxial non-proportional loadings. In this study, a wide range of strain path changes (SPCs) defined as an angle (χ) between stress deviators before and after the SPC is experimentally applied to characterize the anisotropic hardening of a Q&P steel with a strength grade of 980 MPa (QP980). The investigated SPCs include uniaxial tension followed by compression (UT-UC; χ=180°), uniaxial compression followed by tension (UC-UT; χ=180°), uniaxial tension followed by plane strain (UT-PS; χ=30°), equi-biaxial tension followed by plane strain (EBT-PS; χ=30°), followed by uniaxial tension (EBT-UT; χ=60°) and followed by simple shear (EBT-SH; χ=90°). These SPCs are achieved by the program-controlled biaxial tensile testing equipment using novel arm-reinforced cruciforms. The experimental findings in the QP980 steel sheet are summarized as follows: (1) Strong strain-dependent Bauschinger effect, transient hardening and permanent softening are observed in both UT-UC and UC-UT loadings; (2) Bauschinger effect increases as strain increases, and it exhibits obvious tension-compression asymmetry; (3) The transient hardening behaviors are path-dependent under SPCs with various χ; (4) The Bauschinger coefficient is approximately a parabola function of cosχ; (5) The most significant strength softening occurs under EBT-SH loading. In order to describe the complex hardening behavior of the QP980 steel, a distortional hardening model is proposed on the basis of non-associated flow rule. A coupled quadratic and stress-invariant-based yield function is used to describe stress anisotropy, strength differential effect, and anisotropic hardening for proportional loadings. Then, the Bauschinger effect related hardening and path-dependent permanent softening are captured by a modified homogeneous anisotropic hardening model. The calibrated model parameters and experimental validations demonstrate that the strain- and path-dependent Bauschinger effect can be well predicted by the proposed constitutive model. Especially, the model enables to accurately describe both the asymmetric anisotropic hardening and yield surface distortions of the QP980 steel at various biaxial SPCs.

Influence of loading orientation on mechanical properties of spot welds

The mechanical performance and failure characteristics of resistance spot welds from two grades of third-generation advanced high strength steels (3G-AHSS), designated 3G-980 and 3G-1180, to combined/mixed loading were investigated by conducting KS-II tests in eight different loading orientations. Due to inherent difficulty in gripping the high strength 3G-AHSS, a combination of coupon rotation, slippage, and deformation occurred especially at shear-dominated loading orientations. This study utilized a new fixture based on Arcan-apparatus coupled with digital image correlation technique to track the instantaneous orientation of the KS-II coupons. This way, the actual proportion of shear and tensile components of force applied to the nugget, instead of the nominal ratio at the beginning of the tests, could be quantified experimentally, which in turn improved the calibration accuracy of a spot weld strength-based failure criterion. A novel triangulation method was proposed to minimize the influence of coupon slippage and deformation at regions away from the nugget on calculated absorbed energy. The more precise absorbed energy calculation was achieved by changing the displacement reference point from the crosshead to a local measurement between coupon halves. Energy absorption capabilities of the spot welds were calculated by separately tracking the deformation of the nugget in the shear and tensile directions. It was observed that within the tensile-dominated loading orientations (45°–90°) pullout failures of 3G-980 spot welds exhibit much higher post-failure energy absorption capabilities than the RSW of 3G-1180. The propagation of the formed cracks into the fusion zone was found responsible for the relatively inferior energy absorption capability of 3G-1180 joints.

Inclusion Engineering in Medium Mn Steels: Effect of Hot-Rolling Process on the Deformation Behaviors of Oxide and Sulfide Inclusions

Medium Mn steel (MMS) is a new category of the third-generation advanced high strength steel (3rd AHSS) which is developed in the recent 1-2 decades due to a unique trade-off of strength and ductility. Thus, this steel grade has a wide application potential in different fields of industry. The current work provides a fundamental study of the effect of hot-rolling on the inclusion deformation in MMS including a varied 7 to 9 mass pct Mn. Specifically, the deformation behavior of different types of inclusions (i.e., Mn(S,Se), liquid oxide (MnSiO3), MnAl2O4, and complex oxy-sulfide) was investigated. The results show that both MnSiO3 and Mn(S,Se) are soft inclusions which are able to be deformed during the hot-rolling process but MnAl2O4 does not. The aspect ratio of soft inclusions increases significantly from as-cast to hot-rolling conditions. When the maximum size of different inclusions is similar, Mn(S,Se) deforms more than MnSiO3 does. This is due to a joint influence of physical parameters including Young’s modulus, coefficient of thermal expansion (α), etc. However, when the maximum size of one type of inclusion (e.g., MnSiO3) is much larger than another one (e.g., Mn(S,Se)), this maximum size of soft inclusions plays a dominant role than other factors. In addition, the deformation behavior of dual-phase inclusion depends on the major phase, i.e., either oxide or sulfide. Last but not least, empirical correlations between the reduction ratio of the thickness of plate, grain size, and aspect ratio of oxide and sulfide inclusions after hot-rolling are provided quantitatively. This work aims to contribute to the ‘inclusion engineering’ concept in the manufacturing of new generation AHSS.

Damage Analysis of Third-Generation Advanced High-Strength Steel Based on the Gurson–Tvergaard–Needleman (GTN) Model

The third generation of advanced high-strength steels (AHSS) brought attention to the steel and automotive industries due to its good compromise between formability and production costs. This work evaluated a third-generation AHSS (USS CR980XG3TM) through microstructural and X-ray diffraction (XRD) analyses, uniaxial tensile and plane-strain tension testing, and numerical simulations. The damage behavior of this steel is described with the Gurson–Tvergaard–Needleman (GTN) model using an identification procedure based on the uniaxial tensile and initial microvoids data. The microstructure of the CR980XG3TM steel is composed of ferrite, martensite–austenite islands, and retained austenite with a volume fraction of 12.2%. The global formability of the CR980XG3TM steel, namely the product of the uniaxial tensile strength and total elongation values, is 24.3 GPa%. The Lankford coefficient shows a weak initial plastic anisotropy of the CR980XG3TM steel with the in-plane anisotropy close to zero (−0.079) and the normal anisotropy close to unity (0.917). The identified GTN parameters for the CR980XG3TM steel provided a good forecast for the limit strains defined according to ISO 12004-2 standard from the uniaxial tensile and plane-strain tension data.

In Situ Synchrotron X-ray Diffraction and Microstructural Studies on Cold and Hot Stamping Combined with Quenching & Partitioning Processing for Development of Third-Generation Advanced High Strength Steels

A novel combined process of Cold Stamping (CS) and Hot Stamping (HS) with Quenching and Partitioning (Q&P) treatment applied to advanced TRIP-assisted steel has been conducted by thermomechanical simulation to evaluate the influence of CS or HS in the Q&P processing. With this purpose, Q&P, CSQ&P, and HSQ&P cycles were designed to obtain multiphase microstructures containing ferrite, martensite, bainitic-ferrite, and the maximum retained austenite (RA) fraction after the processes. The objective was to investigate the effects of the variables involving the heat treatments, such as the intercritical austenitization temperature, the isothermal and non-isothermal deformation, the amount of deformation, and the temperature and partitioning times, and to analyze their influence on the microstructural and mechanical responses. Time-resolved X-ray diffraction using synchrotron radiation was undertaken in a thermomechanical simulator coupled to the synchrotron light source to understand the influence of time, temperature, and strain on the level of carbon enrichment in austenite. In addition, the in situ austenite transformation kinetics and lattice parameter evolution were tracked, making it possible to optimize the RA fraction at room temperature after Q&P processing. The newly developed combined process is promising as the transformation-induced plasticity phenomenon during deformation can contribute to the formability and energy absorption. The results also indicate that the deformation of austenite promotes the ferrite transformation while suppressing the bainite transformation. It was possible to plot the results in an elongation-mechanical strength diagram, coupled to material property charts, also known as, ‘banana curve’, allowing us to identify and correlate the thermal or thermomechanical treatment conditions that led to an increase in ductility or strength according to the volume fractions of the resulting phases. Comparing the results for the HSQ&P treatments, it was observed that isothermal strains at higher temperatures (≥800 °C) are more advantageous to increase mechanical strength, while non-isothermal strains (starting at 750 °C) are suggested if the objective is the increase in ductility, with mechanical strength being slightly sacrificed.

Deformation mechanism and microstructure evolution of medium-Mn AHSS under various loading conditions

Third-generation advanced high-strength steels possess an exceptional combination of high strength and ductility owing to their high work-hardening rate during deformation. In this study, the mechanical properties and microstructure/texture evolution of medium-Mn steel under various loading conditions (uniaxial tension, in-plane torsion, and equibiaxial tension) were investigated. Interrupted electron back-scattered diffraction and X-ray diffraction characterizations were carried out to track the microstructure and texture evolution of individual phases. Digital image correlation was employed for the in-situ measurement of formation and expansion of deformation bands, i.e., the Lüders and the Portevin-Le Châtelier (PLC) bands. The dynamic strain aging effect in terms of the formation of PLC bands and stress serrations was significantly affected by the loading conditions and stress level. Multiple types of stress serrations were observed in all tested specimens at different deformation stages. The texture evolution of the steel under various deformation modes was further tracked. Ferrite and fresh martensite exhibited the plain texture evolution like most body-centered cubic metals, whereas austenite showed a more complex texture evolution. The martensitic transformation notably decreased the RD||<001> intensity of the austenite; austenite debris was rotated with ferrite grains, while some intact austenite grains self-rotated to a stable orientation, which is unfavorable for martensitic transformation. The obvious intragranular orientation gradient observed in the austenite intact grains implies that considerable deformation occurred in the austenite. Finally, the experimental results indicate that equibiaxial tension primarily promoted the martensitic transformation, and the classical Olson–Cohen model was calibrated to capture the volume fraction evolution of austenite under various stress states.

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.

Adjustment of Mechanical Properties of Medium Manganese Steel Produced by Laser Powder Bed Fusion with a Subsequent Heat Treatment.

Medium manganese steels can exhibit both high strength and ductility due to transformation-induced plasticity (TRIP), caused by metastable retained austenite, which in turn can be adjusted by intercritical annealing. This study addresses the laser additive processability and mechanical properties of the third-generation advanced high strength steels (AHSS) on the basis of medium manganese steel using Laser Powder Bed Fusion (LPBF). For the investigations, an alloy with a manganese concentration of 5 wt.% was gas atomized and processed by LPBF. Intercritical annealing was subsequently performed at different temperatures (630 and 770 °C) and three annealing times (3, 10 and 60 min) to adjust the stability of the retained austenite. Higher annealing temperatures lead to lower yield strength but an increase in tensile strength due to a stronger work-hardening. The maximum elongation at fracture was approximately in the middle of the examined temperature field. The microstructure and properties of the alloy were further investigated by scanning electron microscopy (SEM), hardness measurements, X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and element mapping.

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.

Temperature Effects on Tensile Deformation Behavior of a Medium Manganese TRIP Steel and a Quenched and Partitioned Steel

Third-generation advanced high-strength steels (AHSS) containing metastable retained austenite are being developed for the structural components of vehicles to reduce vehicle weight and improve crash performance. The goal of this work was to compare the effect of temperature on austenite stability and tensile mechanical properties of two steels, a quenched and partitioned (Q&amp;P) steel with a martensite and retained austenite microstructure, and a medium manganese transformation-induced plasticity (TRIP) steel with a ferrite and retained austenite microstructure. Quasi-static tensile tests were performed at temperatures between −10 and 85 °C for the Q&amp;P steel (0.28C-2.56Mn-1.56Si in wt.%), and between −10 and 115 °C for the medium manganese TRIP steel (0.14C-7.14Mn-0.23Si in wt.%). X-ray diffraction measurements as a function of strain were performed from interrupted tensile tests at all test temperatures. For the medium manganese TRIP steel, austenite stability increased significantly, serrated flow behavior changed, and tensile strength and elongation changed significantly with increasing temperature. For the Q&amp;P steel, flow stress was mostly insensitive to temperature, uniform elongation decreased with increasing temperature, and austenite stability increased with increasing temperature. The Olson–Cohen model for the austenite-to-martensite transformation as a function of strain showed good agreement for the medium manganese TRIP steel data and fit most of the Q&amp;P steel data above 1% strain.

Non-metallic Inclusion Evolution in a Liquid Third-Generation Advanced High-Strength Steel in Contact with Double-Saturated Slag

Non-metallic Inclusion Evolution in a Liquid Third-Generation Advanced High-Strength Steel in Contact with Double-Saturated Slag

Role of Intercritical Annealing in Enhancing the Cross-Tension Property of Resistance Spot-Welded Medium Mn Steel

Among the third-generation advanced high-strength steels, medium Mn steels are one of the most promising candidates in the modern automotive industry. However, their application is challenged by the weldability and corresponding poor mechanical property. In this study, the resistance spot welding (RSW) technique was used to join 7Mn steels. A post-weld heat treatment containing intercritical annealing was followed to tune the nugget microstructure. In this case, the trapped C and Mn during RSW can fully diffuse, diminishing the grain boundary embrittlement. Meanwhile, the austenite reversion transformation occurs in the joint, facilitating the formation of a mixed microstructure consisting of ferrite and austenite. With these efforts, the cross-tension strength (CTS) of the RSW 7Mn steel can reach as high as 11.6 kN with outstanding plastic deformation, giving rise to a button pullout fracture mode. This study provides an accessible approach to tailor the microstructure and final CTS of RSW joints of the medium Mn steel, which can guide the optimization of the RSW process.

Isotope Exchange Measurements of the Interfacial Reaction Rate Constant of Nitrogen on Fe-Mn alloys and an Advanced High-Strength Steel

Precise control of dissolved nitrogen is required for producing Third-Generation Advanced High-Strength Steels (AHSS). Compared with well-documented thermodynamic data, the data on the kinetics of nitrogen dissolution in these steels are limited. The isotope exchange technique (15N–14N exchange reaction) was applied to measure the nitrogen reaction rate at different temperatures, on pure liquid iron, liquid iron containing different levels of manganese, and liquid Fe-Mn-Al-Si steel with a Third-Generation AHSS composition. The increase in the interfacial reaction rate with increased temperature corresponded to an activation energy of 144.2 kJ/mol for pure iron. For Fe-Mn alloy, the interfacial reaction rate increased with the increased temperature and increased manganese content. For liquid AHSS (Fe-Mn-Al-Si), the interfacial reaction rate was approximately one third of the rate for pure liquid iron, at the same temperature, with a corresponding activation energy of 160.5 kJ/mol.

Coupled Temperature-Microstructure Model for Predicting Temperature Distribution and Phase Transformation in Steel for Arbitrary Cooling Curves

Abstract This study aims at developing a numerical model that can be employed for simulating the thermomechanical treatment to develop the advanced high strength steels. The developed numerical method is used to calculate the heat transfer coefficient of the quenching medium during the continuous cooling of the steel using the inverse heat transfer model for predefined cooling paths. Further, the phase transformation models are used to predict the final microstructure of the steel plate. The cooling rate, plate thickness, and rolling speed are varied to evaluate the temperature and microstructure distribution in the steel plate. It is found that on increasing the quenching time, the transformation fraction from austenite to ferrite and bainite phases increases and the corresponding martensite fraction decreases. The temperature variation in the plate is significant due to the change in plate thickness and rolling speed for a given quenching time. The present model will be useful for designing process parameters to obtain desired microstructures in third-generation advanced high strength steels.

Effect of two-stage press blanking on edge stretchability with third-generation advanced high-strength steels

Two-stage punching has been recently developed to reduce the sheared edge sensitivity of advanced high-strength steels by dramatically improving the hole expansion ratio via reduction of internal confinement. However, there is still no way to effectively apply this concept for the large-scale press blanking process. In this paper, a novel two-stage press blanking process is proposed to selectively improve the edge stretchability of third-generation advanced high-strength steels for regions with concentrated edge tensioning. Pre-hole design concepts were newly adopted for selective improvement. In the two-stage blanking with pre-holes, inducing the uniform shearing angle is critical for reducing work hardening and ensuring high sheared edge quality. The process is governed by the design and composition of the pre-hole for the first blanking and specifying the lengths of the blanking gap for the second blanking. The proposed two-stage blanking has been experimentally validated by sheared edge tensioning, sheared edge quality, and microhardness tests. A comparison with waterjet cutting showed that it was possible to achieve remarkable edge stretchability that was similar to that obtained for waterjet cutting.

Evaluation of in-plane edge stretchability under severe contact condition for third-generation advanced high-strength steel

This paper newly proposes a practical example to evaluate the edge stretchability of third-generation advanced high-strength steel and conventional GPa-grade steels under an in-plane edge stretching when blanking holding force is applied. Edge stretchability of third-generation advanced high-strength steel was evaluated by a sheared edge tension test in terms of a strain at failure, work hardening, and fracture morphology. An edge stretching mechanism in B-pillar was examined by the numerical simulation, which is applied to design the practical example. To maximize the edge stretching without an undesired fracture excluding the sheared edge, new design concept for the draw-beads was systematically implemented in a proposed practical example for effectively controlling the material inflow. Experimental validations of the practical example were carried out under three different sheared edge conditions produced through flat blanking, humped bottom blanking, and waterjet cutting. These were verified by comparing the sheared edge tension tests with utilizing a digital image correlation technique in terms of strain at the failure.

The Dependence of Fracture Resistance on the Size and Distribution of Blocky Retained Austenite-Martensite Constituents

Although third-generation advanced high-strength steels have achieved a great combination of high ultimate tensile strength and ductility, edge cracking has been frequently reported during their cold forming of components. Following this issue, the effects of the stability, the morphology, and the amount of retained austenite (RA) on fracture resistance have already been investigated. However, the influence of the size and distribution of blocky RA and/or martensite (RA/M) islands on fracture resistance is absent, which is deliberately addressed in this study. After austenitization, the effect of holding temperatures between 278 °C and 400 °C on microstructure evolution in a Fe-0.3C-2.5Mn-1.5Si-0.8Cr (wt pct) steel is systematically and quantitatively investigated. Tensile properties and fracture resistance are characterized using uniaxial tension and double edge-notched tension tests, which interestingly show that an increased product of ultimate tensile strength and total elongation is accompanied with a decreased fracture resistance. This is because that tensile properties are mainly affected by the stability and amount of RA while the fracture resistance is also affected by the size and distribution of blocky RA/M islands. A coarse size of and a small interspacing between blocky RA/M islands are detrimental to the fracture resistance due to the promotion of crack nucleation and crack propagation.

Behavior of ultrafast annealed advanced high strength steels under static and dynamic conditions

Ultrafast annealing (UFA) is considered a feasible alternative processing route for third-generation advanced high strength steels (AHSSs). In fact, significant grain refinement occurs in low carbon steels (0.1 wt%) when heating rates (example, 1000 K/s) considerably higher than the conventional heating rates are applied. Additionally, high heating rates result in multiphase microstructures containing ferrite, retained austenite, and martensite/bainite. The mixture of structural constituents is induced by carbon gradients resulting from the short time available for diffusion during ultrafast annealing of the steel. Quasi-static and high strain rate tensile tests, with strain rates ranging from 0.0033 s−1 to 600 s−1, revealed that the tensile strength of both conventional and UFA samples increases with increasing strain rates, whereas the strain to fracture decreases. However, compared with the conventionally heat-treated samples (heating rate: 10 K/s), the UFA samples exhibited less ductility deterioration at high strain rates. The samples were investigated using light optical microscopy, scanning electron microscopy, electron backscatter diffraction (EBSD), and X-ray diffraction. The results indicated that the improved tensile properties of the UFA steels (compared with those of the conventional steels) resulted mainly from the presence of retained austenite (up to 5%) in conjunction with a lower carbon martensite/bainite fraction. Owing to adiabatic heating associated with dynamic conditions, the stability of retained austenite shifted to higher strain values than those encountered under static conditions. The TRIP effect of the UFA samples appeared at higher strain values than in the conventionally heat-treated samples. The delayed transformation is considered essential for realizing improved preservation of the deformation capacity and, in turn, improved crashworthiness.

Mechanical property development of a 0.15C–6Mn–2Al–1Si third-generation advanced high strength steel using continuous galvanizing heat treatments

The effects of starting microstructure and intercritical annealing (IA) temperature on the mechanical properties of a prototype 0.15C–6Mn-1.5Al–1Si (wt%) third-generation medium manganese advanced high strength steel (3G AHSS) were determined. The general 3G AHSS mechanical property targets UTS × TE of 24,000–40,000 MPa% were met after a continuous galvanizing line (CGL)-compatible IA treatments at 710 °C and 740 °C for 120 s from an 80% martensite – 20% ferrite starting microstructure due to the as-annealed microstructure having large volume fractions of stable retained austenite. The deformation-induced retained austenite to martensite transformation – or TRansformation-induced plasticity (TRIP) – kinetics were a strong function of IA temperature and resulting retained austenite stability. In particular, the 665 °C intercritical anneal produced retained austenite which only partially transformed during deformation and the 740 °C intercritical anneal produced retained austenite which exhausted the TRIP effect at significantly lower strains than those observed for the 710 °C treatment. It was also shown that significant deformation-induced mechanical twinning occurred in the retained austenite, aiding in sustaining the work hardening rate. Overall, it was determined that medium-Mn 3G AHSS can be produced using CGL-compatible thermal cycles, which holds promise for cost-effective production of these alloys.

Austenite Stability and Strain Hardening in C-Mn-Si Quenching and Partitioning Steels

Quenching and partitioning (Q&P) processing of third-generation advanced high strength steels generates multiphase microstructures containing metastable retained austenite. Deformation-induced martensitic transformation of retained austenite improves strength and ductility by increasing instantaneous strain hardening rates. This paper explores the influence of martensitic transformation and strain hardening on tensile performance. Tensile tests were performed on steels with nominally similar compositions and microstructures (11.3 to 12.6 vol. pct retained austenite and 16.7 to 23.4 vol. pct ferrite) at 980 and 1180 MPa ultimate tensile strength levels. For each steel, tensile performance was generally consistent along different orientations in the sheet relative to the rolling direction, but a greater amount of austenite transformation occurred during uniform elongation along the rolling direction. Neither the amount of retained austenite prior to straining nor the total amount of retained austenite transformed during straining could be directly correlated to tensile performance. It is proposed that stability of retained austenite, rather than austenite volume fraction, greatly influences strain hardening rate, and thus controls strength and ductility. If true, this suggests that tailoring austenite stability is critical for optimizing the forming response and crash performance of quenched and partitioned grades.