Abstract The aim of this work was to study systematically the effects of composition and processing on austenite grain growth and static recrystallization (SRX) kinetics, austenite decomposition under controlled cooling as well as microstructures, mechanical properties and weldability of hot rolled low carbon bainitic (LCB) steels. The results showed that the coarsening of austenite grain structure is influenced by the chemical composition. Steels with Nb-Ti alloying exhibited fine and uniform austenite grain size up to 1125 °C, whereas higher temperatures led to formation of the bimodal grain structures. However, with Nb-Ti-B microalloying, the abnormal grain growth was already observed at 1050 °C. SRX rate at roughing temperatures, determined by the stress relaxation method, was found to be retarded markedly by Mo, Nb and B alloying. For the test conditions investigated, the decomposition of austenite started in the temperature range from 780 °C to below 550 °C. All alloying elements with the exception of Nb (0.04–0.10 wt-%) decreased the phase transformation temperatures and increased the hardness of dilatometric specimens. Detailed microstructural examinations enabled the identification of 4 different ferrite morphologies: polygonal ferrite, quasi-polygonal ferrite (QF), granular bainitic ferrite (GB) and bainitic ferrite (BF), generally as a mixed microstructure consisting of 2–3 types morphologies. Consistent with the microstructures detected in dilatometric experiments, the microstructures of rolled plates comprise various combinations of low C ferrite morphologies. These microstructure types provided the yield strengths from 500 MPa up to 850 MPa in hot rolled condition and from 500 MPa to 950 MPa in heat-treated condition (600 °C/1h). The yield strengths from 500 MPa to 570 MPa were mainly related to QF microstructures in as-rolled condition, while the steels with the yield strength from 570 to 700 MPa had GB-QF microstructures. Steels with the yield strengths above 700 MPa consisted of BF. The most effective alloying element regarding the strength properties is B. Also C, Mn, Cr, Mo and Ni have strong influences, but Nb in the range of 0.05–0.10 wt-% is ineffective. Strengthening with B and Mo was detrimental to toughness. Alloying with Ni and Mn is beneficial to good strength and toughness combination. Mn, Mo, Nb and B contents mainly dictate CGHAZ toughness.

The present study focuses on the evolving microstructure of Mg alloy AZ31. The material is subjected to channel die compression at room temperature to simulate a reduction stage in the rolling process. Samples are annealed to provoke recovery, static recrystallization, and grain growth. Annealing is carried out at three temperatures for times ranging from 10s to 10,000s. The material’s response is exhibited through the use of data collection methods such as microhardness, optical microscopy, and electron backscatter diffraction (EBSD). Methodology behind experimentation and data collection techniques are documented in detail. Conclusions are made about the effects of the compression and annealing processes on the material’s microstructure. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) model is introduced, and a simple recrystallization kinetics plot is attempted.

L'objectif de ce travail est de prédire les évolutions microstructurales se produisant dans le tantale pur lors d'un traitement thermique en fonction de son état microstructural initial. La restauration, la recristallisation et la croissance de grains sont décrites à l'aide d'un modèle en champ moyen qui nécessite une description adéquate de la microstructure, en termes de distributions de tailles de grains et de densités de dislocations équivalentes. La densité de dislocation équivalente moyenne peut être évaluée par une simple mesure de dureté Vickers. L'établissement de la relation dureté-densité de dislocations nécessite l'utilisation d'une loi de comportement basée sur la densité de dislocations équivalente. Les évolutions microstructurales au cours d'un traitement thermique ont été observées et les paramètres pilotant ces phénomènes ont été identifiés à l'aide d'essais originaux comme l'observation in situ de la recristallisation ou l'utilisation d'essais à gradient de déformation pour déterminer le seuil de densité de dislocations équivalente pour déclencher la recristallisation. Des essais plus classiques ont permis d'obtenir des cinétiques de recristallisation dans la gamme 1000°C-1100°C pour différentes microstructures initiales. Les simulations des différents traitements thermiques à l'aide du modèle à champ moyen rendent bien compte des évolutions microstructurales en termes de fraction recristallisée et de taille des grains recristallisés pour des microstructures faiblement déformées ou fortement déformées et fragmentées, en utilisant une description adéquate du type de microstructure initiale. Le modèle devra en revanche être adapté pour traiter le cas de microstructures intermédiaires, en enrichissant non seulement la description de la microstructure initiale mais également celle de l'étape de germination des grains recristallisés. Il deviendra alors capable de prédire les évolutions de microstructures pour tout type de microstructure initiale de tantale. / This study aims at predicting the microstructural evolution of pure tantalum during annealing according the initial microstructural state. Static recovery and discontinuous recrystallization as well as grain growth are described using a mean-field model requiring an appropriate description of the microstructure, using both equivalent dislocation densities and grain sizes distributions. The average equivalent dislocation density can be assessed from Vickers microhardness measurements. The calibration of such a relation between microhardness and dislocation density involves the use of a dislocation density-based constitutive law. Microstructural evolutions during annealing have been observed and control parameters of these phenomena have been determined using original tests such as in situ observation of the recrystallization process or the use of strain gradient samples to assess the critical dislocation density for the onset of recrystallization. More classical tests have been carried out to get recrystallization kinetics in the range 1000-1100°C for different initial microstructures. Simulations of annealing using the mean-field model adapted for tantalum match the experimental evolution of both recrystallized fraction and recrystallized grain size, in either weakly deformed or severely deformed and fragmented microstructures. On the other hand, the model needs to be further adapted for intermediate microstructures, with both a more elaborate description of the initial microstructure and of the nucleation stage of the recrystallized grains. It will then be suitable to predict evolutions of any initial tantalum microstructure during annealing.

This thesis contains the development of a physically-based model describing the microstructural evolution during hot deformation of microalloyed steels. The work is mainly focused on the recrystallization kinetics. During hot rolling, the repeated deformation and recrystallization provides progressively refined recrystallized grains. Also, recrystallization enables the material to be deformed more easily and knowledge of the recrystallization kinetics is important in order to predict the required roll forces. Hot strip rolling is generally conducted in a reversing roughing mill followed by a continuous finishing mill. During rolling in the roughing mill the temperature is high and complete recrystallization should occur between passes. In the finishing mill the temperature is lower which means slower recrystallization kinetics and partial or no recrystallization often occurs. If microalloying elements such as Nb, Ti or V are present, the recrystallization can be further retarded by either solute drag or particle pinning. When recrystallization is completely retarded and strain is accumulated between passes, the austenite grains will be severely deformed, i.e. pancaking occurs. Pancaking of the grains provides larger amount of nucleation sites for ferrite grains upon transformation and hence a finer ferrite grain size is achieved. In this work a physically-based model has been used to describe the microstructural evolution of austenite. The model is built-up by several sub-models describing dislocation density evolution, recrystallization, grain growth and precipitation. It is based on dislocation density theory where the generated dislocations during deformation provide the driving force for recrystallization. In the model, subgrains act as nuclei for recrystallization and the condition for recrystallization to start is that the subgrains reach a critical size and configuration. The retarding effect due to elements in solution and as precipitated particles is accounted for in the model. To verify and validate the model axisymmetric compression tests combined with relaxation were modeled and the results were compared with experimental data. The precipitation sub-model was verified by the use of literature data. In addition, rolling in the hot strip mill was modeled using process data from the hot strip mill at SSAB Strip Products Division. The materials investigated were plain C-Mn steels and Nb microalloyed steels. The results from the model show good agreement with measured data. / QC 20100706

<p>Recrystallization kinetics, during and after hot deformation, has been investigated for decades. From these investigations several equations have been derived for describing it. The equations are often empirical or semi-empirical, i.e. they are derived for certain steel grades and are consequently only applicable to steel grades similar to these. To be able to describe the recrystallization kinetics for a variety of steel grades, more physically based models are necessary.</p><p>During rolling in hot strip mills, recrystallization enables the material to be deformed more easily and knowledge of the recrystallization kinetics is important in order to predict the required roll forces. SSAB Tunnplåt in Borlänge is a producer of low-carbon steel strips. In SSAB’s hot strip mill, rolling is conducted in a reversing roughing mill followed by a continuous finishing mill. In the reversing roughing mill the temperature is high and the inter-pass times are long. This allows for full recrystallization to occur during the inter-pass times. Due to the high temperature, the rather low strain rates and the large strains there is also a possibility for dynamic recrystallization to occur during deformation, which in turn leads to metadynamic recrystallization after deformation. In the finishing mill the temperature is lower and the inter-pass times are shorter. The lower temperature means slower recrystallization kinetics and the shorter inter-pass times could mean that there is not enough time for full recrystallization to occur. Hence, partial or no recrystallization occurs in the finishing mill, but the accumulated strain from pass to pass could lead to dynamic recrystallization and subsequently to metadynamic recrystallization.</p><p>In this work a newly developed physically based model has been used to describe the microstructural evolution of austenite. The model is based on dislocation theory where the generated dislocations during deformation provide the driving force for recrystallization. The model is built up by several submodels where the recrystallization model is one of them. The recrystallization model is based on the unified theory of continuous and discontinuous recovery, recrystallization and grain growth by Humphreys.</p><p>To verify and validate the model, rolling in the hot strip mill was modeled using process data from SSAB’s hot strip mill. In addition axisymmetric compression tests combined with relaxation was modeled using experimental results from tests conducted on a Gleeble 1500 thermomechanical simulator at Oulu University, Finland. The results show good agreement with measured data.</p>

Recrystallization kinetics, during and after hot deformation, has been investigated for decades. From these investigations several equations have been derived for describing it. The equations are often empirical or semi-empirical, i.e. they are derived for certain steel grades and are consequently only applicable to steel grades similar to these. To be able to describe the recrystallization kinetics for a variety of steel grades, more physically based models are necessary. During rolling in hot strip mills, recrystallization enables the material to be deformed more easily and knowledge of the recrystallization kinetics is important in order to predict the required roll forces. SSAB Tunnplåt in Borlänge is a producer of low-carbon steel strips. In SSAB’s hot strip mill, rolling is conducted in a reversing roughing mill followed by a continuous finishing mill. In the reversing roughing mill the temperature is high and the inter-pass times are long. This allows for full recrystallization to occur during the inter-pass times. Due to the high temperature, the rather low strain rates and the large strains there is also a possibility for dynamic recrystallization to occur during deformation, which in turn leads to metadynamic recrystallization after deformation. In the finishing mill the temperature is lower and the inter-pass times are shorter. The lower temperature means slower recrystallization kinetics and the shorter inter-pass times could mean that there is not enough time for full recrystallization to occur. Hence, partial or no recrystallization occurs in the finishing mill, but the accumulated strain from pass to pass could lead to dynamic recrystallization and subsequently to metadynamic recrystallization. In this work a newly developed physically based model has been used to describe the microstructural evolution of austenite. The model is based on dislocation theory where the generated dislocations during deformation provide the driving force for recrystallization. The model is built up by several submodels where the recrystallization model is one of them. The recrystallization model is based on the unified theory of continuous and discontinuous recovery, recrystallization and grain growth by Humphreys. To verify and validate the model, rolling in the hot strip mill was modeled using process data from SSAB’s hot strip mill. In addition axisymmetric compression tests combined with relaxation was modeled using experimental results from tests conducted on a Gleeble 1500 thermomechanical simulator at Oulu University, Finland. The results show good agreement with measured data. / QC 20101118

Abstract In this thesis, methods to process ultrafine ferritic (UFF) structures in steels, i.e. grain sizes below about 3 μm have been investigated. It is shown here, in accordance with the results in the literature, that a steel with a UFF grain size can be obtained by two methods, more or less convenient to mass production: deformation-induced ferrite transformation from fine-grained austenite (the DIF route) and the static recrystallization of various heavily cold-worked initial microstructures (the SRF/SRM route). In the present work, the influencing factors in the processing of UFF structure in the DIF route have been systematically studied in four low-carbon steels: one C-Mn steel and Nb, Nb-Ti and Nb-high Ti microalloyed steels. A high strain, a low deformation temperature close to Ar3 and a fine prior austenite grain size are beneficial to promote the formation of UFF grains. Especially by using complex pretreatments to refine the prior austenite grain size, cold rolling, repeating the low-temperature reheating cycle or using martensitic initial microstructure, a UFF grain size can be obtained in these steels at the strain of 1.2 (70% reduction) at 780 °C. By controlling the cooling rate, the type of the second phase can be adjusted. When using the static recrystallization route, it was found that UFF is difficult to obtain from a single-phase ferrite, but it is relatively readily obtained from deformed pearlite, bainite or martensite, especially in high-carbon steels with 0.3–0.8%C. In deformed pearlite, the cementite lamellae fragmented and spheroidised in the course of heavy deformation can provide numerous nucleation sites by the particle stimulated nucleation mechanism and retard the subgrain and recrystallized grain growth. Nucleation and retardation of grain growth are effective also in deformed bainite, martensite or high-carbon tempered martensite, as discussed in detail in the work. The thermal stability of UFF grained steels was tested and found to be generally excellent, but it varies depending on the processing method. The UFF structure obtained by the SRM route has a thermal stability somewhat weaker than that of the DIF route. For a given steel, UFF grains may show different grain growth modes, related to the dispersion of second phase particles. In the DIF structure, abnormal grain growth occurs at 700 °C after about 2.5 h, while in the SRM structure, normal grain growth takes place slowly at 600 °C. Carbides on the grain boundaries seem to play an important role in inhibiting grain coarsening. No coarse-grained zone was formed at the HAZ of electron beam or laser welded seams, as performed at low heat inputs (up to 1.5 kJ/cm) on thin strips. The hardness even increased from the base metal towards the HAZ and the weld metal in all seams as an indication that they were hardened during the rapid cooling.

Aluminiumschichten werden durch Kaltgasspritzen auf fünf verschiedene poly- und monokristalline keramische Werkstoffe (Al2O3 , AlN, SiC, Si3N4 , MgF2 ) appliziert. Dabei erfolgt eine Variation der Substrattemperatur und der Partikelgröße. Ausgewählte Proben werden einer nachfolgenden Wärmebehandlung unterzogen. Die im Fokus der Arbeit stehende Erforschung der an der Grenzfläche zwischen Aluminium und Keramik wirkenden Haftmechanismen erfolgt sowohl mithilfe einer mechanischen Charakterisierung (Stirnzugversuche) als auch durch verschiedene mikroskopische, spektroskopische und hochauflösende Methoden. Die Bewertung der Untersuchungsergebnisse zeigt, dass im Allgemeinen ein Anstieg der Haftzugfestigkeit mit steigender Substrat- und Wärmebehandlungstemperatur sowie mit zunehmender thermischer Effusivität des Substratwerkstoffs zu verzeichnen ist. Eine vergleichbare Auswirkung hat innerhalb bestimmter Grenzen die Zunahme der Partikelgröße. Mit der Heteroepitaxie wird neben der mechanischen Verklammerung ein weiterer wichtiger Haftmechanismus kaltgasgespritzter metallischer Schichten auf keramischen Substraten identifiziert. Die Ausbildung von quasiadiabatischen Scherbändern und statische Rekristallisationsprozesse wirken dabei als wichtige begleitende Mechanismen. Als Nachweis für heteroepitaktisches Wachstum ist die Existenz von (annähernd) parallelen, senkrecht oder geneigt zur Grenzfläche stehenden Ebenenpaaren, die eine geringe Gitterfehlanpassung aufweisen, zu werten. Der Vergleich mit PVD-Schichten zeigt, dass in Bezug auf die Orientierung von Gitterebenen verschiedene Mechanismen der Heteroepitaxie existieren, die von der atomaren Mobilität des Beschichtungswerkstoffs bestimmt werden.

Aluminiumschichten werden durch Kaltgasspritzen auf fünf verschiedene poly- und monokristalline keramische Werkstoffe (Al2O3 , AlN, SiC, Si3N4 , MgF2 ) appliziert. Dabei erfolgt eine Variation der Substrattemperatur und der Partikelgröße. Ausgewählte Proben werden einer nachfolgenden Wärmebehandlung unterzogen. Die im Fokus der Arbeit stehende Erforschung der an der Grenzfläche zwischen Aluminium und Keramik wirkenden Haftmechanismen erfolgt sowohl mithilfe einer mechanischen Charakterisierung (Stirnzugversuche) als auch durch verschiedene mikroskopische, spektroskopische und hochauflösende Methoden. Die Bewertung der Untersuchungsergebnisse zeigt, dass im Allgemeinen ein Anstieg der Haftzugfestigkeit mit steigender Substrat- und Wärmebehandlungstemperatur sowie mit zunehmender thermischer Effusivität des Substratwerkstoffs zu verzeichnen ist. Eine vergleichbare Auswirkung hat innerhalb bestimmter Grenzen die Zunahme der Partikelgröße. Mit der Heteroepitaxie wird neben der mechanischen Verklammerung ein weiterer wichtiger Haftmechanismus kaltgasgespritzter metallischer Schichten auf keramischen Substraten identifiziert. Die Ausbildung von quasiadiabatischen Scherbändern und statische Rekristallisationsprozesse wirken dabei als wichtige begleitende Mechanismen. Als Nachweis für heteroepitaktisches Wachstum ist die Existenz von (annähernd) parallelen, senkrecht oder geneigt zur Grenzfläche stehenden Ebenenpaaren, die eine geringe Gitterfehlanpassung aufweisen, zu werten. Der Vergleich mit PVD-Schichten zeigt, dass in Bezug auf die Orientierung von Gitterebenen verschiedene Mechanismen der Heteroepitaxie existieren, die von der atomaren Mobilität des Beschichtungswerkstoffs bestimmt werden.