TRIP-assisted Multiphase Steels Research Articles

In-situ neutron diffraction study on the deformation of a TRIP-assisted multi-phase steel composed of ferrite, austenite and martensite

Multi-phase steels showing transformation induced plasticity (TRIP), can exhibit an excellent combination of high strength and good ductility by the aid of martensitic transformation during deformation. Even though TRIP-assisted multi-phase steels have been widely used in industry, the role of each phase in the enhancement of mechanical properties is still unclear given their complicated microstructures. In order to understand better the nature of the TRIP effect, the mechanical interaction between different phases at the micro-scale should be clarified. In the present study, the mechanical behavior of a transformation induced plasticity (TRIP) assisted multi-phase steel, has been characterized by in-situ neutron diffraction during tensile testing. The result of strain partitioning between the different phases obtained from the in-situ neutron analysis revealed that the martensite phase took much more elastic strain than the ferrite and retained austenite phases, which suggests that the work hardening behavior in the present steel is affected by the higher load borne by deformation-induced martensite.

Stress partitioning among ferrite, martensite and retained austenite of a TRIP-assisted multiphase steel: An in-situ high-energy X-ray diffraction study

The stress partitioning among ferrite, martensite and retained austenite, and the stability of retained austenite under uniaxial tensile loading of a TRIP-assisted multiphase steel are investigated using in-situ high-energy X-ray diffraction technique. The results indicate that the stress partitioning, mainly occurring in elastic and elastoplastic regions, is activated by the “stress feedback mechanism”. In the elastic region, the decline of increasing rate of ferrite stress is accompanied by the sudden increase of martensite and retained austenite stresses, which indicates a stress transfer from ferrite to martensite and retained austenite. During deformation, martensitic transformation first occurs in the retained austenite with low carbon content, followed by those with high carbon content, which is denominated “selective transformation mechanism”. In addition, the increment of average carbon content in retained austenite continuously increases with the gradual transformation of retained austenite into martensite. The evolution of retained austenite strain versus engineering stress is similar to that of martensite strain, except that austenite strain is higher than martensite strain at the end of uniform deformation, which is related to the high carbon content in retained austenite.

Towards ultra-high ductility TRIP-assisted multiphase steels controlled by strain gradient plasticity effects

TRansformation Induced Plasticity (TRIP) is a very effective mechanism to increase the strain hardening capacity of multiphase steels containing a fraction of metastable austenite, leading to both high strength and large uniform elongation. Excellent performances have been reached in the past 20 years, with recent renewed interest through the development of the 3rd generation of high strength steels often involving a TRIP effect. The microstructure and composition optimization is complex due to the interplay of coupled effects on the transformation kinetics and work hardening such as phase stability, size of retained austenite grains, temperature and loading path. In particular, recent studies have shown that the TRIP effect can only be quantitatively captured for realistic microstructures if strain gradient plasticity effects are taken into account, although direct experimental validation of this claim is missing. Here, an original computational averaging scheme is developed for predicting the elastoplastic response of TRIP aided multiphase steels based on a strain gradient plasticity model. The microstructure is represented by an aggregate of many elementary unit cells involving each a fraction of retained austenite with a specified stability. The model parameters, involving the transformation kinetics, are identified based on experimental tensile tests performed at different temperatures. The model is further assessed towards original experiments, involving temperature changes during deformation. A classical size independent plasticity model is shown unable to capture the TRIP effect on the mechanical response. Conversely, the strain gradient formulation properly predicts substantial variations of the strain hardening with deformation and temperature, hence of the uniform elongation in good agreement with the experiments. A parametric study is performed to get more insight on the effect of the material length scale as well as to determine optimum transformation kinetics to reach the highest possible strength-ductility balance. It is shown that the uniform elongation can potentially be increased by 50% or more, paving the way towards future microstructure engineering efforts.

Beneficial influence of an intercritically rolled recovered ferritic matrix on the mechanical properties of TRIP-assisted multiphase steels

The present study deals with the microstructure and mechanical properties of intercritically rolled TRIP-assisted multiphase steels. It is shown that the occurrence of the TRIP effect in a recovered ferritic matrix brings about an improved strength–ductility balance with respect to a fully recrystallised ferrite matrix. On the other hand, the intercritical deformation does not influence the austenite transformation rate during straining at room temperature. The improvement of the mechanical properties results from the interactions between the transformation strain and the recovered ferrite.

Micromechanical behavior of TRIP-assisted multiphase steels studied with in situ high-energy X-ray diffraction

The stress partitions among multiple phases for two cold-rolled transformation-induced-plasticity (TRIP)-assisted C–Mn–Al–Si steels, with different carbon content levels of 0.1% and 0.2% (mass fractions), were investigated using in situ high-energy X-ray diffraction at ambient and low temperatures (−40°C) under uniaxial tensile loading. Based on the evolution of stresses for various phases during plastic deformation, a modified constitutive model was established for describing the micromechanical behavior of TRIP-assisted multiphase steels, based on a Gladman-type mixture law (GTML) embedded with the Mecking–Kocks work-hardening formula. The index n, an important parameter in the GTML for characterizing the accommodation of loading stresses among different phases, was determined to be severely affected by the transformation kinetics of retained austenite. The quantitative relationship between n and the transformation rate of retained austenite was also clearly revealed in the investigated alloys. The modified model thus correlates the mismatch of stresses for multiple phases with the transformation kinetics of metastable phases during plastic deformation and is suitable for all advanced steels with multi-scale microstructures. This model forms the basis for microstructure-based numerical simulations of micromechanical behavior and greatly benefits the design of a new type of high-strength and high-plasticity steel.

Deformation Behaviour of TRIP Steel Monitored by <i>In Situ</i> Neutron Diffraction

The paper presents results of in-situ neutron diffraction experiments aimed on monitoring the phase evolution and load distribution in transformation induced plasticity (TRIP) steel when subjected to tensile loading. Tensile deformation behaviour of two TRIP-assisted multiphase steel with slightly different microstructures resulted from different thermo-mechanical treatments applied was investigated by in-situ neutron diffraction. The steel with lower retained austenite volume fraction (fγ=0.04) and higher volume fraction of needle-like bainite in the α-matrix exhibits higher yield stress (sample B, 600MPa) but considerably lower elongation in comparison to the steel with higher austenite volume fraction (fγ=0.08), granular bainite and ferrite matrix (sample A, 500 MPa). The neutron diffraction results showed that the applied tensile load is redistributed at the yielding point in a way that the retained austenite bears a significantly larger load than the α-matrix during the TRIP steel deformation. Steel sample with higher volume fraction of retained austenite and less strong ferrite matrix proved to be a better TRIP steel with respect to strength, ductility and the side effect of the strain induced austenite-martensite transformation. The transforming retained austenite in time of loading provides potential for higher ductility of experimental TRIP steel but at the same time acts as a reinforcement phase during the further plastic deformation.TRIP steel, austenite conditioning, austenite transformation, structure, retained austenite, tensile deformation, neutron diffraction, load partitioning, mechanical properties.

Characterization of NbC and (Nb, Ti)N nanoprecipitates in TRIP assisted multiphase steels

Multiphase steels utilising composite strengthening may be further strengthened via grain refinement or precipitation by the addition of microalloying elements. In this study a Nb microalloyed steel comprising martensite, bainite and retained austenite has been studied. By means of transmission electron microscopy (TEM) we have investigated the size distribution and the structural properties of (Nb,Ti)N and NbC precipitates, their occurrence in the various steel phases, and their relationship with the Fe matrix. (Nb,Ti)N precipitates were found in ferrite, martensite, and bainite, while NbC precipitates were found only in ferrite. All NbC precipitates were found to be small (5–20nm in size) and to have a face centred cubic (fcc) crystal structure with lattice parameter a=4.36±0.05Å. In contrast, the (Nb,Ti)N precipitates were found to have a broader size range (5–150nm) and to have a fcc crystal structure with lattice parameter a=8.09±0.05Å. While the NbC precipitates were found to be randomly oriented, the (Nb,Ti)N precipitates have a well-defined Nishiyama–Wasserman orientation relationship with the ferrite matrix. An analysis of the lattice mismatch suggests that the latter precipitates have a high potential for effective strengthening. Density functional theory calculations were performed for various stoichiometries of NbCx and NbxTiyNz phases and the comparison with experimental data indicates that both the carbides and nitrides are deficient in C and N content.

Analysis of size effects associated to the transformation strain in TRIP steels with strain gradient plasticity

The size dependent strengthening resulting from the transformation strain in Transformation Induced Plasticity (TRIP) steels is investigated using a two-dimensional embedded cell model of a simplified microstructure composed of small cylindrical metastable austenitic inclusions within a ferritic matrix. Earlier studies have shown that within the framework of classical plasticity or of the single length parameter Fleck–Hutchinson strain gradient plasticity theory, the transformation strain has no significant impact on the overall strengthening. The strengthening is essentially coming from the composite effect with a marked inclusion size effect resulting from the appearance during deformation of new boundaries constraining the plastic flow. The three parameters version of the Fleck–Hutchinson strain gradient plasticity theory is used here in order to better capture the effect of the plastic strain gradients resulting from the transformation strain. The three parameters theory incorporates separately the rotational and extensional gradients in the formulation, which leads to a significant influence of the shear component of the transformation strain, not captured by the single-parameter theory. When the size of the austenitic inclusions decreases, the overall strengthening increases due to a combined size dependent effect of the transformation strain and of the evolving composite structure. A parametric study is proposed and discussed in the light of experimental evidences giving indications on the optimization of the microstructure of TRIP-assisted multi-phase steels.

On measurement of retained austenite in multiphase TRIP steels — results of blind round robin test involving six different techniques

The present paper reports the results of a blind round robin test dedicated to the measurement of the retained austenite content of different TRIP assisted multiphase steels. Various surface and volume techniques, i.e. light microscopy, X-ray diffraction, electron backscattered diffraction (EBSD), magnetic saturation, thermal diffusivity and laser ultrasonics, were used by different partners. The compiled results show a quite large variability of the estimated retained austenite content, particularly for well established techniques, such as X-ray diffraction (XRD) and magnetisation. On the other hand, emerging techniques like EBSD, thermal diffusivity and laser ultrasonics warrant further investigations.

Strain gradient plasticity analysis of transformation induced plasticity in multiphase steels

“To what extent do plastic strain gradients affect the strengthening resulting from the transformation of small metastable inclusions into hard inclusions within a plastically deforming matrix?” is the central question addressed here. Though general in the approach, the focus is on the behavior of TRIP-assisted multiphase steels. A two-dimensional embedded cell model of a simplified microstructure composed of a single metastable austenitic inclusion surrounded by a soft ferritic matrix is considered. The cell is inserted in a large homogenized medium. The transformation of a fraction of the austenite into a hard martensite plate is simulated, accounting for a transformation strain, and leading to complex elastic and plastic accommodation. The size of a transforming plate in real multiphase steels is typically between 0.1 and 2 μm, a range of size in which plastic strain gradient effects are expected to play a major role. The single parameter version of the Fleck–Hutchinson strain gradient plasticity theory is used to describe the plasticity in the austenite, ferrite and martensite phases. The higher order boundary conditions imposed on the plastic flow have a large impact on the predicted strengthening. Using realistic values of the intrinsic length parameter setting the scale at which the gradients effects have an influence leads to a noticeable increase of the strengthening on top of the increase due to the transformation of a volume fraction of the retained austenite. The geometrical parameters such as the volume fraction of retained austenite and of the transforming zone also bring significant strengthening. Strain gradient effects also significantly affect the stress state inside the martensite plate during and after transformation with a potential impact on the damage resistance of these steels.

The fracture toughness of TRIP-assisted multiphase steels

The relationship between microstructure and fracture resistance of TRIP-assisted multiphase steels has been investigated by processing and characterizing eight specifically designed microstructures differing in terms of the volume fractions of the constitutive phases, i.e. ferrite, bainite, retained austenite and martensite, by the stability of the retained austenite, and by the connectivity of the phases. Fracture resistance has been quantified by the fracture strain in uniaxial tension, ε ¯ f , the fracture toughness at cracking initiation, J Ic, and by the essential work of fracture, w e. The fracture mechanisms were characterized by microscopic observations and by profilometric measurements of the crack tip necking process typical in thin sheet fracture. The fracture toughness at cracking initiation of TRIP-aided steels corresponds to an equivalent Dual Phase steel in which martensite replaces retained austenite and is thus not affected by the retained austenite stability. The ductile tearing resistance of TRIP-aided steels, quantified by w e, increases significantly with the retained austenite stability. This improved tearing resistance is explained by the effect of the additional work-hardening induced by the late transformation of retained austenite within the crack tip necking zone.

Tensile properties of different chemical compositions for TRIP-assisted multiphase steel for automobile structures

The microstructural changes and the tensile properties of TRIP-assisted steels resulting from different chemical compositions were investigated by using SEM, TEM, XRD and UTM. As a result of microscopic observation, the morphology of retained austenite could be characterized by two types: a granular type in steel containing higher Si and a film type in steel having higher C. In the case of the steel containing higher C with a tensile strength of 860 MPa and a total elongation of 38%, the film type retained austenite could be observed among the lath bainitic ferrites. Actually, the metastable retained austenite was required for good formability, which means that the chemical composition plays a significant role in the microstructure and tensile property of TRIP-assisted steel. With respect to the tensile property, each steel type that contained an suitable amount of Si and Mn demonstrated a typical TRIP effect on a stress-strain curve while steel that contained a higher Mn content exhibited similar behaviors, as demonstrated in the dual phase steels.

Multiscale mechanics of TRIP-assisted multiphase steels: I. Characterization and mechanical testing

The mechanical behaviour of transformation-induced plasticity (TRIP)-assisted multiphase steels is addressed based on three different microstructures generated from the same steel grade. The mechanisms responsible for the work-hardening capacity and the resulting balance between strength and resistance to plastic localization are investigated at different length scales. The macroscopic mechanical response is determined by simple shear, uniaxial tension, Marciniak and equibiaxial tension supplemented by earlier tensile tests on notched and cracked specimens. It is shown that the transformation rate reaches a maximum for stress states intermediate between uniaxial tension and equibiaxial tension. At an intermediate length scale, the true in situ flow properties of the individual ferrite–bainite and retained austenite phases are determined by combining neutron diffraction and digital image correlation. This combined analysis elucidates the partitioning of stress and strain between the different constitutive phases. Based on these results, supplemented by transmission electron microscopy and electron backscattered diffraction observations, a general overview of the hardening behaviour of TRIP-assisted multiphase steels is depicted.

On the Phase Transformations in Hot Rolled TRIP-Assisted Multiphase Steels

TRIP-assisted multiphase steels exhibit an excellent balance of strength and ductility, which makes them very attractive for the automotive industry. These remarkable mechanical properties can be attributed mainly to their composite-like microstructures and to the transformation of retained austenite into martensite during straining (Transformation-Induced Plasticity). The aim of this study is to highlight the interactions between the hot rolling conditions, the transformation of austenite and formation of the microstructure, and the resulting mechanical properties. Various rolling simulation techniques were employed to determine how the composite microstructure is formed during the various steps of multi-stage thermomechanical processing.

Interactions between Recrystallisation and Phase Transformations during Annealing of Cold Rolled Nb-Added TRIP-Aided Steels

Fe and Fe-C based alloys present the exceptional feature that the processing route can be adapted to lead to various phases that present antagonist mechanical properties ranging from soft ferrite to high strength martensite. Among the different deformation mechanisms that can be exhibited by these phases, the TRIP effect brings about large enhancements of the work-hardening rate. The current TRIP-assisted multiphase steels present a ferrite-based matrix with a distribution of islands of bainite and retained austenite obtained at the end of specific thermal or thermomechanical treatments. The present study aims at characterising the interactions occurring between ferrite recrystallisation and austenite formation during the intercritical annealing of cold rolled Nb-added TRIP-aided steels. It is shown that the addition of niobium retards the ferrite recrystallisation during heating. As a consequence, ferrite may not be completely recrystallised before the nucleation and growth of the austenite grains. Strong interactions between these phenomena can then be observed, i.e. a strong hindering of the ferrite recrystallisation due to the austenite formation. Furthermore, the heating rate from room temperature to the intercritical temperature range influences the thermodynamic conditions prevailing at the ferrite / austenite interface and dictates the phase proportions.

Three-dimensional computational-cell modeling of the micromechanics of the martensitic transformation in transformation-induced-plasticity-assisted multiphase steels

A three-dimensional finite-element microstructural cell model involving an inclusion of retained austenite embedded within a ferrite grain, which is surrounded by a homogeneous matrix representing the behavior of a transformation-induced-plasticity (TRIP)-assisted multiphase steel, was developed in order to address the micromechanics of the martensitic transformation in small isolated austenite grains. The transformation of a single martensite plate is simulated after various amounts of prior plastic deformation under different in-plane loading conditions. The values of the mechanical driving force and of the elastic and plastic accommodation energies associated with the transformation are calculated as a function of the externally applied loading conditions. The mechanical driving force and the total accommodation energy are of the same order of magnitude. The mechanical driving force depends upon the stress state and is the highest for plane-strain conditions. The total accommodation energy is almost independent of the stress state. It is affected by the amount of plastic straining prior to transformation and is very much dependent on the level of the shear component of the transformation strain. The results of this study provide guidelines for the development of realistic stress-state-dependent transformation evolution laws for TRIP-assisted multiphase steels.

TRIP-Assisted Multiphase Steels with Niobium Additions. Microstructures and Properties

It seems completely clear that the combination within the same microstructure of various steel phases (ferrite, bainite, martensite) and the existence of the TRIP effect (i.e. a mechanically induced martensitic transformation) improves in a large way the properties of strength and ductility of low-alloy steels. TRIP-aided multiphase steels have been studied for several years and arouse a growing industrial interest. This project deals with the influence of a large Nb addition on the microstructure and mechanical properties of TRIP-assisted multiphase steels. It is shown that Nb influences in a large way the different phase transformations as well as the recrystallisation of the ferrite matrix occurring during the heat-treatment of cold-rolled TRIP-aided steels. As a consequence, the mechanical properties also depend on the parameters of the thermomechanical process and on the content and stability of the retained austenite.

Critical Assessment of the Micromechanical Behaviour of Dual Phase and Trip-Assisted Multiphase Steels

The strength and formability properties of high performance steels have been recently improved thanks to the combination of several strengthening mechanisms such as dislocation strengthening and mechanically-induced martensitic transformation within complex microstructures. These finely grained metastable microstructures are generated during controlled multistage thermomechanical treatments involving several phase transformations. However, the way the different active mechanisms combine in order to improve the properties of these steels is not yet clearly established. The present study proposes an overview of an experimental program devoted to the micromechanical characterisation of Dual Phase and TRIP-assisted multiphase steels. Thanks to different techniques such as X-Ray and neutron diffraction, strain mapping and transmission electron microscopy (TEM), it was possible to characterize the flow behaviour of the different constitutive phases and to measure the critical parameters of the microstructures responsible for the work hardening capabilities of the TRIP-assisted multiphase steels. Comparison of the experimental stress and strain partitioning within the heterogeneous microstructures with the predictions of different modelling assumptions was also carried out.La résistance mécanique et les propriétés de formabilité des aciers à haut performance ont été améliorées récemment grâce à la combinaison de plusieurs mécanismes de renforcement comme le renforcement par dislocations et la transformation martensitique induite mécaniquement dans des microstructures complexes. Ces microstructures métastables à grains fins sont engendrées lors de traitements thermomécaniques contrôlés, à plusieurs étapes, impliquant plusieurs transformations de phase. Cependant, la manière dont les différents mécanismes actifs se combinent pour améliorer les propriétés de ces aciers n’est pas encore clairement établie. Cette présente étude propose une revue du programme expérimental dévoué à la caractérisation micromécanique des aciers biphasés et des aciers multiphasés à effet TRIP. Grâce à différentes techniques comme la diffraction des rayons X et des neutrons, la cartographie de la déformation et le TEM, il a été possible de caractériser les propriétés d’écoulement des différentes phases constitutives et de mesurer les paramètres microstructuraux critiques responsables de la capacité d’écrouissage des aciers multiphasés à effet TRIP. On effectue également une comparaison du partitionnement expérimental de la contrainte et de la déformation à l’intérieur des microstructures hétérogènes avec les prédictions de différentes hypothèses de modélisation.

Critical Assessment of the Micromechanical Behaviour of Dual Phase and Trip-Assisted Multiphase Steels

The strength and formability properties of high performance steels have been recently improved thanks to the combination of several strengthening mechanisms such as dislocation strengthening and mechanically-induced martensitic transformation within complex microstructures. These finely grained metastable microstructures are generated during controlled multistage thermomechanical treatments involving several phase transformations. However, the way the different active mechanisms combine in order to improve the properties of these steels is not yet clearly established. The present study proposes an overview of an experimental program devoted to the micromechanical characterisation of Dual Phase and TRIP-assisted multiphase steels. Thanks to different techniques such as X-Ray and neutron diffraction, strain mapping and transmission electron microscopy (TEM), it was possible to characterize the flow behaviour of the different constitutive phases and to measure the critical parameters of the microstructures responsible for the work hardening capabilities of the TRIP-assisted multiphase steels. Comparison of the experimental stress and strain partitioning within the heterogeneous microstructures with the predictions of different modelling assumptions was also carried out.La résistance mécanique et les propriétés de formabilité des aciers à haut performance ont été améliorées récemment grâce à la combinaison de plusieurs mécanismes de renforcement comme le renforcement par dislocations et la transformation martensitique induite mécaniquement dans des microstructures complexes. Ces microstructures métastables à grains fins sont engendrées lors de traitements thermomécaniques contrôlés, à plusieurs étapes, impliquant plusieurs transformations de phase. Cependant, la manière dont les différents mécanismes actifs se combinent pour améliorer les propriétés de ces aciers n’est pas encore clairement établie. Cette présente étude propose une revue du programme expérimental dévoué à la caractérisation micromécanique des aciers biphasés et des aciers multiphasés à effet TRIP. Grâce à différentes techniques comme la diffraction des rayons X et des neutrons, la cartographie de la déformation et le TEM, il a été possible de caractériser les propriétés d’écoulement des différentes phases constitutives et de mesurer les paramètres microstructuraux critiques responsables de la capacité d’écrouissage des aciers multiphasés à effet TRIP. On effectue également une comparaison du partitionnement expérimental de la contrainte et de la déformation à l’intérieur des microstructures hétérogènes avec les prédictions de différentes hypothèses de modélisation.

Experimental investigation of the influence of the stress state on the mechanical stability of austenite in multiphase steels

The tranformation-induced plasticity (TRIP) effect, i.e. the mechanically activated martensitic transformation of metastable austenite, has been proven for some years to contribute very effectively to the deformation process in a large variety of iron-based alloys. In order to enlighten the influence of the stress triaxiality on the kinetics of the mechanically-induced martensitic transformation in TRIP-assisted multiphase steels. several specimens presenting austenite with different mechanical stabilities were strained under different stress states (pure uniaxial tension, uniaxial tension of notched and DENT specimens and stretching). It is shown that the stress triaxiality has a large effect on the mechanical stability of austenite dispersed in a multiphase microstructure and consequently on the mechanical properties of the investigated steels.