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The main purpose of this thesis is to improve the fundamental knowledge of non-isothermal tempering of martensite phase and its effects on the reduction in hardness (softening) with respect the base metal occurring at the heat affected zone (HAZ) of resistance spot welded dual-phase (DP) steels. This thesis also aims at understanding the influence of HAZ-softening on the joint performance of various DP steel grades. The tempering of martensite occurring at the sub-critical HAZ (SC-HAZ) of resistance spot welded DP600, DP780 and DP980 steels has been systematically evaluated by microhardness testing through Vickers indentation and the degree of tempering has been correlated to the HAZ-softening. From the joint performance analysis of similar and dissimilar steel grade combinations assessed through standardized testing methods, three important issues have been targeted: a) the joint strength (maximum load to failure), b) the location of failure (failure mode), and c) the physical characteristic of the weld that determines certain type of failure (weld nugget size). In addition, a partial tensile test has been conducted in order to evaluate the initiation of failure in dissimilar steel grade combinations. It has been shown that HAZ-softening lowered the weld size at which transition from interfacial to pullout failure mode takes place along with increased load-bearing capacity and higher energy absorption. Thus, it is concluded from mechanical testing that HAZ-softening benefits the lap-shear tensile joint performance of resistance spot welded DP steels by facilitating pullout failures through failure initiation at the SC-HAZ (tempered region). Instrumented nanoindentation testing was employed to further investigate HAZ-softening along the SC-HAZ by evaluating individual phases of ferrite matrix and tempered martensite islands. Although the ferrite matrix presented a slight reduction in hardness at nanoscale, higher reduction in hardness (softening) resulted for tempered martensite; thus confirming that tempered martensite is the major contributor to softening at micro-scale. A comparison between nanohardness and microhardness testing made at different distances from the line of lower critical temperature of transformation (Ac1) allowed revealing the actual extension of the SC-HAZ. In this regard, good correlation was obtained between nanohardness results along the SC-HAZ and the microstructural changes analyzed by electron microscopy (i.e., the tempering of martensite occurring at various distances far from Ac1 was correlated to low temperature tempering of dual phase steels). An in-depth analysis of the tempering of martensite phase at high temperature in DP steel subjected non-isothermal conditions i.e., rapid heating, extremely short time at peak temperature and rapid cooling (resistance spot welding), has been carried out mainly through analytical transmission electron microscopy (TEM). In addition, an isothermal tempering condition (i.e., slow heating and long time at peak temperature) in DP steel has been evaluated for complementing the analysis. Both non-isothermal and isothermal conditions have been correlated to the softening behaviour. TEM analysis of the base metal in the DP steel indicated that the morphology of the martensite phase is dependent on its carbon content, and its tempering characteristics are similar to that of equal carbon containing martensitic steel. The isothermally tempered structure is characterized by coarsening and spheroidization of cementite (θ) and complete recovery of the martensite laths; whereas precipitation of fine quasi-spherical intralath θ-carbides, coarser plate-like interlath θ-carbides, decomposition of retained austenite into elongated θ-carbides, and partial recovery of the lath structure were observed after non-isothermal tempering of DP steel. This difference in tempering behaviour is attributed to synergistic effect of delay in cementite precipitation due to higher heating rate, and insufficient time for diffusion of carbon that delays the third stage of tempering process (cementite coarsening and recrystalization) during non-isothermal. The finer size and the plate-like morphology of the precipitated carbides along with the partial recovery of the lath structure observed after non-isothermal tempering strongly influenced the softening behaviour of DP steel. The chemical analysis of θ-carbides through extraction replicas for three different DP steels revealed that the chemistry of the carbides is inherited from the parent DP steel during non-isothermal tempering at high temperature confirming that non-isothermal tempering DP steel is predominantly controlled by carbon diffusion.

Quantitative relationships among processing parameters, microstructure, and material properties are of considerable interest in the context of development of robust processing routes that optimize the required material properties. As a result, the scientific literature contains a large number of experimental and theoretical studies on microstructure-properties relationships. Fracture sensitive mechanical properties such as ductility, ultimate tensile strength, fatigue life, and fracture toughness depend on the average microstructural parameters as well as the distributions of microstructural parameters and their extrema.Development of quantitative relationships between such material properties and microstructural distributions and extrema has received considerably less attention, particularly in the wrought metals and alloys. Accordingly, an important objective of this research is to perform a systematic investigation in this direction. The dependence of the fracture-sensitive mechanical properties on the microstructural distributions and extrema often leads to substantial variability in these properties: a set of specimens having the same average chemistry, the same average processing history, and the same average microstructural parameters such as volume fractions of different constituents can exhibit substantially different material properties. The present research (i) is concerned with high strength (~ 1000 MPa) high martensite (>50%) dual phase steel where the martensite is a topologically continuous phase (matrix) containing a dispersion of islands of ferrite, and (ii) focuses on understanding the microstructural origins of the variability in fracture sensitive mechanical properties, in particular variability in the room temperature uniaxial tensile ductility. The research involves quantitative microstructure characterization using stereology and digital image processing and quantitative fractography using scanning electron microscopy (SEM) and fracture profilometry. The analysis of the quantitative fractographic and microstructural data obtained in this research leads to useful guidelines for reducing the variability in the tensile ductility of the dual phase steel under investigation.

The main purpose of this thesis is to improve the fundamental knowledge of non-isothermal tempering of martensite phase and its effects on the reduction in hardness (softening) with respect the base metal occurring at the heat affected zone (HAZ) of resistance spot welded dual-phase (DP) steels. This thesis also aims at understanding the influence of HAZ-softening on the joint performance of various DP steel grades. The tempering of martensite occurring at the sub-critical HAZ (SC-HAZ) of resistance spot welded DP600, DP780 and DP980 steels has been systematically evaluated by microhardness testing through Vickers indentation and the degree of tempering has been correlated to the HAZ-softening. From the joint performance analysis of similar and dissimilar steel grade combinations assessed through standardized testing methods, three important issues have been targeted: a) the joint strength (maximum load to failure), b) the location of failure (failure mode), and c) the physical characteristic of the weld that determines certain type of failure (weld nugget size). In addition, a partial tensile test has been conducted in order to evaluate the initiation of failure in dissimilar steel grade combinations. It has been shown that HAZ-softening lowered the weld size at which transition from interfacial to pullout failure mode takes place along with increased load-bearing capacity and higher energy absorption. Thus, it is concluded from mechanical testing that HAZ-softening benefits the lap-shear tensile joint performance of resistance spot welded DP steels by facilitating pullout failures through failure initiation at the SC-HAZ (tempered region). Instrumented nanoindentation testing was employed to further investigate HAZ-softening along the SC-HAZ by evaluating individual phases of ferrite matrix and tempered martensite islands. Although the ferrite matrix presented a slight reduction in hardness at nanoscale, higher reduction in hardness (softening) resulted for tempered martensite; thus confirming that tempered martensite is the major contributor to softening at micro-scale. A comparison between nanohardness and microhardness testing made at different distances from the line of lower critical temperature of transformation (Ac1) allowed revealing the actual extension of the SC-HAZ. In this regard, good correlation was obtained between nanohardness results along the SC-HAZ and the microstructural changes analyzed by electron microscopy (i.e., the tempering of martensite occurring at various distances far from Ac1 was correlated to low temperature tempering of dual phase steels). An in-depth analysis of the tempering of martensite phase at high temperature in DP steel subjected non-isothermal conditions i.e., rapid heating, extremely short time at peak temperature and rapid cooling (resistance spot welding), has been carried out mainly through analytical transmission electron microscopy (TEM). In addition, an isothermal tempering condition (i.e., slow heating and long time at peak temperature) in DP steel has been evaluated for complementing the analysis. Both non-isothermal and isothermal conditions have been correlated to the softening behaviour. TEM analysis of the base metal in the DP steel indicated that the morphology of the martensite phase is dependent on its carbon content, and its tempering characteristics are similar to that of equal carbon containing martensitic steel. The isothermally tempered structure is characterized by coarsening and spheroidization of cementite (θ) and complete recovery of the martensite laths; whereas precipitation of fine quasi-spherical intralath θ-carbides, coarser plate-like interlath θ-carbides, decomposition of retained austenite into elongated θ-carbides, and partial recovery of the lath structure were observed after non-isothermal tempering of DP steel. This difference in tempering behaviour is attributed to synergistic effect of delay in cementite precipitation due to higher heating rate, and insufficient time for diffusion of carbon that delays the third stage of tempering process (cementite coarsening and recrystalization) during non-isothermal. The finer size and the plate-like morphology of the precipitated carbides along with the partial recovery of the lath structure observed after non-isothermal tempering strongly influenced the softening behaviour of DP steel. The chemical analysis of θ-carbides through extraction replicas for three different DP steels revealed that the chemistry of the carbides is inherited from the parent DP steel during non-isothermal tempering at high temperature confirming that non-isothermal tempering DP steel is predominantly controlled by carbon diffusion.

This thesis is devoted to the numerical investigation of mechanical behavior of Dual phase (DP) steels. Such grade of advanced high strength steels (AHSS) is favorable to the automotive industry due the unique properties such as high strength and ductility with low finished cost. Many experimental and numerical studies have been done to achieve the optimized behavior of DP steels by controlling their microstructure. Experiments are costly and time consuming so in recent years numerical tools are utilized to help the metallurgist before doing experiments. Most of the numerical studies are based on classical (local) constitutive models where no material length scale parameters are incorporated in the model. Although these models are proved to be very effective in modeling the material behavior in the large scales but they fail to address some critical phenomena which are important for our goals. First, they fail to address the size effect phenomena which materials show at microstructural scale. This means that materials show stronger behavior at small scales compared to large scales. Another issue with classical models is the mesh size dependency in modeling the softening behavior of materials. This means that in the finite element context (FEM) the results will be mesh size dependent and no converged solution exist upon mesh refinement. Thereby by applying the classical (local) models one my loose the accuracy on measuring the strength and ductility of DP steels. Among the non-classical (nonlocal) models, gradient-enhanced plasticity models which consider the effect of neighboring point on the behavior of one specific point are proved to be numerically effective and versatile tools to accomplish the two concerns mentioned above. So in this thesis a gradient-enhanced plasticity model which incorporates both the energetic and dissipative material length scales is derived based on the laws of thermodynamics. This model also has a consistent yield-like function for the interface which is an essential part of the higher-order gradient theories. The main issue with utilizing these theories is the implementation which limits the application of these theories for modeling the real problems. Here a straightforward implementation method based on the classical FEM and Meshless method will be proposed which due to its simplicity it can be applied for many problems. The application of the developed model and implementation will be shown on removing the mesh size dependency and capturing the size effect in microstructure level of dual phase steels.

Bead-on-plate laser welds were made on an industrially produced DP780 steel to determine the effect of normalized welding heat input on the microstructure and mechanical properties within the weld fusion zone (FZ) and heat affected zone (HAZ) with reference to the base material (BM) mechanical properties. Normalized welding heat input was calculated using an established model from the literature utilizing measurements from the weld cross-section microstructures along with known materials properties. Microhardness profiles and optical microscopy were employed to evaluate materials properties and microstructural changes across the various microstructural zones of each weld. The mechanical properties of the welds were evaluated globally through standard ASTM tensile specimens as well as through a series of specialized mechanical testing sample geometries which examined the properties of individual microstructural zones. These specialized sample geometries included non-standard uniaxial and plain strain tension where effective stress and effective strains were used to compare the mechanical properties across samples. It was determined that there was a good correlation between ASTM standard samples and the specialized sample geometries employed in this study and that the UTS and YS values obtained in both cases were comparable. Sigmoidal decay behaviour was observed in the UTS and YS with increasing heat input for both the FZ and HAZ of all welds. It was found that welds with heat inputs greater than 60 J/mm2 had both a UTS and YS which were significantly depressed in the FZ and HAZ when compared to the base material values. Conversely, welds with heat inputs below 36.3 J/mm2 were found to have a UTS and YS in both the FZ and HAZ microstructural zones which were above the values determined for the BM. When global weld properties were tested, it was found that welds with a heat input greater than 60.0 J/mm2 failed within the HAZ while welds with heat inputs below 36.3 J/mm2 failed within the BM. It has been shown that there is a significant correlation between the heat inputs of laser welded DP steels and both the mechanical properties and microstructural features of the various microstructural zones as well as the location of failure during weld tensile testing. It has also been demonstrated that the mechanical properties of weld microstructural zones can be qualitatively evaluated using specialized tensile testing geometries. / Thesis / Master of Applied Science (MASc)

<p> Intercritically annealed steels, such as dual-phase steels, have found widespread use in automotive structural components due to their high strength and ductility. Elements such as Mn, Al and Si, added to improve the mechanical properties are selectively oxidized during heat treatment and limit the ability of the alloy to be reactively wet during continuous hot-dip galvanizing. Subsequently, a limit has been placed on the amount of alloy which can be used if the steel is to be subsequently galvanized. The specifics of this limit have not been explored in detail, nor has the mechanism of decreased wettability been well demonstrated in the literature other than to say that the galvanizing reaction is limited by oxides on the surface.</p> <p> Using a force balance, it is shown that the presence of MnO on the surface of steels greatly reduces the wettability with a typical galvanizing bath (Zn-0.2wt%Al, Fe-saturated, 460°C). Furthermore, it was determined that this is caused by the additional and rate-limiting step of aluminothermic reduction of the oxide layer with the bath Al, required for subsequent inhibition layer formation. By using a low pO2 during annealing, the wettability was improved by reducing the thickness of the MnO layer when compared to intermediate and industrially common values of pO2. Using a high pO2 also resulted in improved wettability since the internal oxide which was formed did not reduce the wettability since it was not exposed to the bath alloy.</p> <p> Improvements in hardenability were also explored via dilatometry showing that the formation of bainite is delayed with increasing Mn content, as well as a decrease in transformation temperatures from γ during cooling (i.e. Ms and Bs). At ~5wt% Mn, only the the transformation to αM could be observed. This opens the door to higher strength, galvanized steels - as well as possibly galvanized martensitic steels.</p> / Thesis / Master of Applied Science (MASc)

This study is mainly aimed at interstital-free and dual-phase steels, analyzing the compositions and distribution of selective surface oxides after annealing and then to know the influence of these oxidation for the formation of FeAl inhibition layer in hot-dip galvanizing. Interstital-free and dual-phase steels were first annealed at 800 oC for 1-200 s in a 10% H2-N2 protected atmosphere of -70 oC and 0 oC dew point respectively and then dipped in zinc bath with Al content 0.12-0.18 wt% for 0-20 s. Using this combined SEM, Auger electron spectroscopy(AES), X-ray photoelectron spectroscopy(XPS) and ICP-AES etc. instruments, it is shown that the MnAl2O4 spinels were the major oxidation on the surface of IF steel after annealing. The average oxidation thickness was about 5-15 nm. Annealing times has little effect on the thickness. On the other hand, MnO were observed on DP steel surface after anneaing. The MnO paticles mainly distributed at the grain boundaries ,and the average oxdaiton thickness increase rapidly from 20 nm(10 s) to 110 nm(200 s) with annealing times. The growth of the FeAl inhibition layer can separate to nucleation in initial stage and diffusion growth later. The Fe2Al5 nucleation times were all about 0.1 s in both steels , and average thicknesses were approximately 20 nm. For IF steels , Al uptake in the zinc bath and steel interface was depleted in nucleation stage with 0.12 wt% Al content, so that delayed the growth of Fe2Al5, and the rate determining step was the diffusion of Al in zinc bath. When Al content raise up to 0.14 wt%, the phenomenon of growth delay was not happened, and the rate determining step of Fe2Al5 growth changed to the solid-state diffusion of Fe in Fe2Al5. For DP steels, when Al content up to 0.14 wt%, the growth mechanism was similar to IF steels, but the rate determining step of Fe2Al5 growth was mainly in the grain boundary diffusion of Fe in Fe2Al5. Moreover, where the MnO paticles was rich could obviously observe the delay of Fe2Al5 growth. It was probably because of consuming a great deal of Al to reduce the MnO oxides.

Les aciers Dual-Phase sont largement utilisés dans le secteur de l’automobile enraison de leurs propriétés mécaniques remarquables et du bon compromis résistanceductilité qui lui donne d’intéressante potentialités comme absorbeur d’énergiemécanique. Cependant, la recherche de bons compromis entre les propriétésmécaniques en traction et celles de formabilité nécessite une optimisation desparamètres microstructuraux. Ce travail de thèse s’inscrit dans cet optique. Dans unepremière partie, l’étude bibliographique proposée permet de mieux cerner lesparamètres influençant la formation des microstructures ainsi que les propriétés desaciers DP. Dans une seconde partie, nous proposons un travail expérimental originalpermettant de mieux comprendre la formation des microstructures des aciers DP etde découpler l’effet de certains paramètres microstructuraux sur les propriétés deces aciers. Enfin, la modélisation micromécanique proposée permet de compléter etd’interpréter les données expérimentales acquises. Ce travail ouvre des voiesintéressantes de « design » des microstructures des aciers DP en vue de développerdes aciers de nouvelles générations possédant des propriétés optimisées. / Ferrite-martensite dual-phase (DP) steels have been widely used in automotiveindustry due to their excellent mechanical properties, such as high work-hardeningrate and a good compromise between strength and ductility allowing high energyabsorbing performance. In order to fully exploit the potential of DP steels and extendthe application, the dual-phase microstructure has to be optimized for bettercombination of strength and formability that is characterized by uniform strainand/or fracture strain. As a starting point, detailed literature review is made on themicrostructure development and mechanical properties of DP steels, and the keyfactors controlling microstructural features and determining mechanical propertiesare identified. Through experimental investigation, microstructures are developed inorder to decouple the effects of various microstructural features, and themicrostructure—mechanical properties relationship is systematically studied.Micromechanical modeling is used to further understand the experimental resultswithin a quantitative framework, and to provide a support for microstructurerefinement of DP steels by parametric study. Strategies of designing DP steels tofulfill specific forming operation have been proposed, and the concept of DP steelswith graded martensite islands has been discussed with FEM analysis as a possibilityof improving strength—formability trade-off.