The object of this study was to investigate the micro-structural response of DP 600 subjected to high strain rate, ballistic impact tests. The ballistic tests were conducted using normal impact of a hardened steel penetrator into a 2 mm thick sheet of DP 600. The average strain rates produced from this test method are on the order of 10^5 s-1. Multiple methods were used to investigate the micro-structure before and after high strain rate deformation including optical microscopy, electron microscopy, and X-ray diffraction. A large variation in material response was observed between tests conducted at 0.8 x 10^5 and 2.5 x 10^5 s-1.

<p>Advanced high strength steels, such as dual-phase and martensitic steels, are increasingly being used by automakers to decrease the thickness of steel sheet used in parts without sacrificing part strength. When welded, the martensite within the dual-phase and martensitic steel microstructures tempers, reducing the heat-affected zone (HAZ) hardness compared to the base material, locally reducing strength. This process is known as HAZ softening. HAZ softening has been well studied; however, the kinetics of this process has not been quantified and the processes responsible for HAZ softening have not been examined. This thesis investigated both of these topics.</p> <p>HAZ softening was modelled using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. As the thermal profile during welding is non-isothermal, the effects of temperature and time on steel tempering kinetics could not be separated by examining post-welded properties. The effects of tempering temperature and time were separated through a series rapid isothermal tempering experiments. Hardness data from these experiments allowed the HAZ softening rate to be empirically quantified through fitting the JMAK equation. This material model was then validated by predicting HAZ softening in laser and resistance spot welds. Although the fitted JMAK constants could be used to predict post-weld HAZ hardness, they did not agree with the classic literature values associated with martensite tempering.</p> <p>To understand why the JMAK coefficients did not match those of the classic martensite tempering literature, the softening data from one of the martensitic steels was re-examined. This study revealed that the softening process was a combination of two processes: carbide nucleation and carbide coarsening. The activation energies calculated for each process matched the classic literature values. Carbide coarsening dominated during tempering, which had a non-linear relation with change in hardness. The relationship between carbide coarsening and hardness was responsible for the softening kinetics measured from the rapid tempering experiments.</p> / Doctor of Philosophy (PhD)

Dual-Phase (DP) steels, part of the group of Advanced High Strength Steels (AHSS), are used by car manufactures due to its large strength to weight ratio. The high strength of the DP steel does have a negative impact on the formability during sheet metal forming and stretch forming, e.g. fractures often appear in shear cut edges during forming of blanks made of DP steel.   The main objective with this thesis is to develop a new punch for Volvo Cars that concentrates the strain to the sheared edges of a test specimen made from different types of DP steel. This is done to be able to measure and obtain maximum fracture strain during stretch forming tests in a press. The newly developed test method is called CTEST (Concentrated Trim Edge Strain Test).   The tests are performed with DP steel specimens with three different qualities of the shear cut edges; fine cut, medium cut and worn cut. DP steels tested are DP600GI, DP600UC and DP800GI from three different suppliers. 10 different types of DP steels are tested in this study with different thickness. Thickness of specimens tested are 1 mm, 1.1 mm, 1.5 mm and 2 mm and all specimens tested have a lengthwise (RD) rolling direction.   The quality of the sheared cut edge has a great impact to the formability and maximum fracture strain of the specimen. A specimen with a fine cut endures higher fracture strain than medium cut and a worn cut for all types of DP steel with different thickness. A 1 mm thick specimen endures a lower fracture strain than 1.5 mm and 2 mm specimen for all cut qualities.   Further, the impact of the orientation of the burr zone of a shear cut edge is studied. With the burr zone facing upwards from the CTEST punch the formability of the specimens is decreased compared to a burr zone facing downwards, especially for a worn cut specimen with micro cracks and imperfections in the edge surface.   ARAMIS Digital Image Correlation (DIC) system is used to analyze the specimen edges during press experiments. The ARAMIS results unveil that several small fractures appear in the sheared edges of a specimen just before the specimens split into two pieces. This phenomenon was seen for specimen with worn and medium shear cut qualities.   Finite Element (FE) simulations of the CTEST is performed in AutoForm to determine maximum values of the true strain for the three different cut qualities. The simulation in AutoForm does show a slightly higher value of the force and press depth than the value from the press test before maximum fracture strain in reached. The small fractures seen in ARAMIS just before the specimen split into two pieces cannot be seen in the simulation in AutoForm.

Fiber laser welding (FLW) was used to join advanced high strength steel (AHSS) and high strength steel (HSS); specifically two dual-phase (DP) steels, with ultimate tensile strengths above 980 MPa and with different chemistries (DP980 Rich and DP980 Lean), and a high strength low alloy (HSLA) steel, with an ultimate tensile strength of 450MPa (HSLA450). The welding speed and power were varied to develop a process envelope for minimizing weld concavity. In order to attain welds free of weld concavity a balance of speed and power was required; weld concavity could be reduced by lowering power and increasing speed. Welds with amounts of concavity ranging from 15 % to 35 % were characterized with respect to hardness, tensile and fatigue testing. Tensile results revealed that DP steel was sensitive to weld concavity while HSLA450 was not. At stress amplitudes enduring beyond 1000 cycles, welded specimens exhibited lower fatigue resistance compared to the base metal. Concavity reduced the fatigue life of DP980 steels, where increasing the amount of concavity further reduced the fatigue resistance, while the fatigue resistance of HSLA steel welds was not sensitive to weld concavity. Hardness profiling of the welds revealed that HAZ softening was present in the DP980 steel welds. The amount of HAZ softening was normalized; allowing for comparison of different steels. Welds made by FLW demonstrated reduced softening compared other laser welding types because FLW was capable of welding with lower heat input. A difference in the FZ hardness was observed between the DP980 steels because of the difference in carbon content of the steels; where higher carbon content resulted in higher FZ hardness. Additionally the high cooling rate in FLW created higher fusion zone hardness than the values predicted by Yurioka’s model based on arc welding. Examination of the microstructure revealed that the soft zone of DP980 Lean steel possessed severely tempered martensite and untransformed ferrite while DP980 Rich generated a structure with a mixture of tempered martensite, untransformed ferrite and a small fraction of non-tempered martensite. This difference in HAZ softening was attributed to the alloying content of the DP980 Rich steel the higher alloying content of DP980 Rich steel formed a stable austenite that could exist near the Ac1 temperature and enabled the formation of new martensite in the soft zone. The effects of HAZ softening were apparent in tensile testing where the DP980 Lean steel, which exhibited higher softening, demonstrated by a severe reduction in elongation while the DP980 Rich steel, which had higher resistance to softening, attained elongation comparable to its base metal. HSLA450 exhibited a slight reduction in elongation due to the hardening of the fusion zone. The welded DP980 Rich and HSLA450 steels consistently failed within the base metal, while the DP980 Lean steel failed in the soft zone. The welded DP980 Rich steel also demonstrated limiting dome heights comparable to the base metal while the severe softening in the DP980 Lean led to premature fracture in the soft zone, yielding a larger reduction in the limiting dome height.

Fiber laser welding (FLW) was used to join advanced high strength steel (AHSS) and high strength steel (HSS); specifically two dual-phase (DP) steels, with ultimate tensile strengths above 980 MPa and with different chemistries (DP980 Rich and DP980 Lean), and a high strength low alloy (HSLA) steel, with an ultimate tensile strength of 450MPa (HSLA450). The welding speed and power were varied to develop a process envelope for minimizing weld concavity. In order to attain welds free of weld concavity a balance of speed and power was required; weld concavity could be reduced by lowering power and increasing speed. Welds with amounts of concavity ranging from 15 % to 35 % were characterized with respect to hardness, tensile and fatigue testing. Tensile results revealed that DP steel was sensitive to weld concavity while HSLA450 was not. At stress amplitudes enduring beyond 1000 cycles, welded specimens exhibited lower fatigue resistance compared to the base metal. Concavity reduced the fatigue life of DP980 steels, where increasing the amount of concavity further reduced the fatigue resistance, while the fatigue resistance of HSLA steel welds was not sensitive to weld concavity. Hardness profiling of the welds revealed that HAZ softening was present in the DP980 steel welds. The amount of HAZ softening was normalized; allowing for comparison of different steels. Welds made by FLW demonstrated reduced softening compared other laser welding types because FLW was capable of welding with lower heat input. A difference in the FZ hardness was observed between the DP980 steels because of the difference in carbon content of the steels; where higher carbon content resulted in higher FZ hardness. Additionally the high cooling rate in FLW created higher fusion zone hardness than the values predicted by Yurioka’s model based on arc welding. Examination of the microstructure revealed that the soft zone of DP980 Lean steel possessed severely tempered martensite and untransformed ferrite while DP980 Rich generated a structure with a mixture of tempered martensite, untransformed ferrite and a small fraction of non-tempered martensite. This difference in HAZ softening was attributed to the alloying content of the DP980 Rich steel the higher alloying content of DP980 Rich steel formed a stable austenite that could exist near the Ac1 temperature and enabled the formation of new martensite in the soft zone. The effects of HAZ softening were apparent in tensile testing where the DP980 Lean steel, which exhibited higher softening, demonstrated by a severe reduction in elongation while the DP980 Rich steel, which had higher resistance to softening, attained elongation comparable to its base metal. HSLA450 exhibited a slight reduction in elongation due to the hardening of the fusion zone. The welded DP980 Rich and HSLA450 steels consistently failed within the base metal, while the DP980 Lean steel failed in the soft zone. The welded DP980 Rich steel also demonstrated limiting dome heights comparable to the base metal while the severe softening in the DP980 Lean led to premature fracture in the soft zone, yielding a larger reduction in the limiting dome height.

Orientador: Marcelo dos Santos Pereira / Banca: Humberto Lopes Rodrigues / Banca: Pedro Paulo de Campos / Resumo: Estudos de novos materiais, mais resistentes, econômicos e de menor densidade, são de grande interesse para aplicações estruturais em projetos de Engenharias. Propriedades mecânicas antes obtidas principalmente por meio de adição de elementos de ligas, como por exemplo elevada dureza, tenacidade e resistência mecânica, podem ser atingidas atualmente com variações no processamento térmico e termomecânico. Estas alterações têm proporcionado a formação de estruturas multifásicas nos aços, e como elemento deste estudo, os aços bifásicos. Neste trabalho, buscou-se entender os efeitos que processamentos térmicos possuem sobre as microestruturas dos aços de médio teor de carbono (SAE 1045), assim como a sua influência sobre as propriedades mecânicas, especialmente sobre a tenacidade, evidenciada por meio de ensaios de impacto. O material tratado termicamente obteve, sua estrutura transformada, visando à formação de estrutura bifásica (ferrita e martensita) e de estrutura formada por martensita revenida, sendo comparadas com a condição do material como fornecido (ferrita e perlita). A caracterização microestrutural foi realizada para determinação das fases presentes, assim como das frações volumétricas respectivas. Ensaios de tração e de impacto foram realizados visando a determinação das propriedades mecânicas do aço. Os resultados mostraram que a interação entre ferrita e martensita, gerada na condição bifásica, possue forte influência nos resultados referentes à tenacidade, assim como a temperatura em que o material foi ensaiado. Comparando-se três estruturas, comprova-se que a estrutura bifásica possui resistência mecânica e tenacidade mais elevadas que os aços temperados e revenidos, e tenacidade proxima a do aço com médio teor de carbono, sem tratamento térmico. / Abstract: Studies of new materials, more resistance, cheaper costs and lower density, have high interest for structural use in engineering projects. Mechanical properties adquired in the past by alloyes adition, for example igher hardness, toughness and mechanical strenght, nowadays can be reached using controlled thermo mechanical process. These changes have provided the multiphase structures formation in steels, for example the dual phase steel, subject of the present study. In this work it was researched the effects of heat process in microestructure of medium carbon steels (SAE 1045) and the influence over mechanical properties, specially in toughers, realized by impact analysis. The heat treatment material has its structure transformed in dual-phase structure (ferrite and martensite) and compared with supplied material (ferrite and perlite). The microestructural characterization was done to determine the present phase, and the respective volumetric fraction of martensite. Tension and impact analysis were done to determine the steel mechanical properties. Results have shown that the interaction between ferrite and martensite generated in dual-phase condition, has higher influence, related the materials toughness, so the temperature that material was analysed comparing the three structures, verifies that the dual-phase struture has level of mechanical resistence and toughness, higher than quenched and tempered steels, and toughness near of medium carbon steel that has not received heat treatment in higher or lower temperatures. / Mestre

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19893号 / 工博第4209号 / 新制||工||1651(附属図書館) / 32970 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 辻 伸泰, 教授 白井 泰治, 教授 松原 英一郎 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

The processes leading to the retention of small quantities of austenite following the bainite and martensite phase transformations have been examined, together with the influence of retained austenite on the properties of low alloy steels. It was found that the upper and lower bainite transformations are separate reactions, although both involve a displacive transformation mode, Growth seems to occur by the repeated nucleation of martensitic sub-units, and this leads to an apparently slow growth rate, The partitioning of carbon from bainitic ferrite into residual austenite was thermodynamically proven to occur subsequent to transformation, and was shown to be directly responsible for the 'incomplete reaction phenomenon'. The nature of sympathetic nucleation and of the limited size of bainitic sub-units was rationalised in terms of the relatively low driving force available for bainite transformations. It was shown that the retention, stability and morphology of austenite could be directly derived from the basic transformation mechanism. Under certain circumstances, the bainitic retained austenite conferred exceptional strength/toughness properties to silicon steels; these were· shown to be superior to the properties associated with tempered martensite microstructures, Using thermodynamics, a model was established which could predict the toughness behaviour of silicon steel bainites simply from a knowledge of the composition. The tempered martensite embrittlement phenomenon was not found to be directly linked to the decomposition of retained austenite films, but to the coarsening of inter- or intra-lath carbides. In dislocated martensites, it was found that the distribution and quantity of retained austenite could be rationalised in terms of the degree of accommodation between adjacent martensite variants. The incipient twins generally observed in lath martensites were shown to be accommodation defects such that the extent of twinning was the greatest when adjacent martensite units had twin-related lattices. The thermodynamics of dislocated martensites have been briefly examined, The inhomogeneous deformation behaviour of dual-phase steels has been analysed in terms of available models.

Les aciers Dual-Phase (DP) sont des aciers à très haute résistance mécanique (AHSS) fortement utilisés pour des applications automobiles en raison de leur très bon compromis résistance mécanique/ductilité ainsi que par leur habilité à répondre aux multiples exigences industrielles (bas prix, assemblage, revêtement etc.). A l'heure actuelle, le développement d'aciers DP apparait durable pour la caisse-en-blanc des structures automobiles. La microstructure ferrite-martensite, caractéristique des aciers DP, est obtenue par un traitement thermique complexe composé de différentes étapes au cours desquelles différents mécanismes métallurgiques entrent en jeux. Les principales difficultés de production sont liées au fait (i)que les évolutions microstructurales sont influencées par les différents paramètres de traitement thermique (vitesse de chauffe, température ...), (ii) que les différentes étapes de traitement sont interconnectées et que (iii) les évolutions microstructurales peuvent se chevaucher et, part conséquent, interagir entre elles. Ces travaux de thèse ont pour objectif d'améliorer la compréhension des évolutions métallurgiques entrant en jeux lors des traitements thermiques des aciers DP, et notamment d'améliorer la compréhension de l'influence des paramètres de traitement. Les différentes évolutions métallurgiques ont été, dans un premier temps, caractérisées en couplant un ensemble de techniques expérimentales (dilatométrie, dureté, TPE ...) et grâce à un protocole assurant de décorréler les mécanismes se superposant et interconnectés. A titre d'exemple, les deux principaux mécanismes entrant en jeux lors de l'étape de recuit intercritique ont été, dans un premier temps, étudiés séparément ((1) la recristallisation a été étudié en dessous de la température de formation d'austénite et (2) la formation d'austénite a été étudié sur un acier pré-recristallisé) avant de se concentrer sur le cas des aciers laminés à froid où la recristallisation et formation d'austénite se superposent. Le projet avait également pour objectif de développer des outils de prédictions permettant de décrire les évolutions microstructurales basés sur des approches empiriques (loi de JMAK) ainsi que vers des modèles à base plus physique (mixed-mode modèle et modèle diffusif). Une attention particulière a été dédié à discuter de la fiabilité, l'adaptabilité, des forces et limitations des différentes approches développées. / Dual-Phase steels (DP) are one of the most used Advanced High Strength Steels (AHSS) for automotive applications because they present good strength/ductility compromise and they adapt to number of industrial constraints (low price, shaping, welding, coating etc.). Nowadays, the development of DP steels seems to be promising and sustainable for the body-in-white structure. The typical ferrite-martensite microstructure, characteristic of DP steels, are obtained by a thermal treatment composed of different stages during which metallurgical evolutions occur. Major difficulties of their processing are due to the fact that (i) microstructural evolution kinetics are influenced by cycle parameters (heating rate, annealing temperature etc.), (ii) different stages are interconnected and (iii) some microstructural evolutions may overlap and, therefore, interact. This PhD-work aimed at getting a better understanding of microstructural evolutions during the thermal cycle of DP steel and, namely, the influence of cycle parameters. Different microstructural evolutions occurring during the thermal cycle were first characterized coupling different experimental techniques (dilatometry, hardness, TPE etc.) and with a particular protocol in order to decorrelate overlapping and interconnected phenomena. As example, two major evolutions occurring during the intercritical annealing were first studied individually ((1) recrystallization was investigated below austenite formation temperature and (2) austenite formation was investigated on prior recrystallized steels) before investigated cold-rolled steel case where recrystallization and austenite formation overlap. The study was then attached to develop some predictive tools to describe microstructural evolutions based on phenomenological approaches (JMAK law) towards more physical based models (mixed-mode, diffusive models). A particular care was attached to discuss on model reliability, versatility, strengths and limitations.

In der Arbeit werden spezielle Kupferlote im MIG- und Laserlötverfahren an Stählen mit unterschiedlichem Festigkeitsverhalten untersucht. Die Ergebnisse sollen dazu beitragen, den Lötprozess durch den Einsatz spezieller Kupferbasislote zu optimieren und durch reduzierten Energieeintrag ein homogeneres Eigenschaftsfeld im Bereich der Fügestelle zu erzeugen. Den Verarbeitern dieser Werkstoffe soll die Möglichkeit gegeben werden, diese Werkstoffe rationeller und mit höherer Effektivität zu verarbeiten. Im Ergebnis der Arbeit sollen Verbesserungen der Eigenschaften der Lötnähte erzielt werden, die besonders in der Dünnblechverarbeitung mit Schwerpunkt Karosseriebau Anwendung finden. Wesentliche Ziele sind die Erhöhung der Festigkeitseigenschaften, eine Erhöhung der Fügegeschwindigkeit, die Verbesserung des Phosphatierungsverhaltens sowie eine Reduzierung der eingebrachten Wärmeenergie. Die Vielfältigkeit dieser Anforderungen macht es notwendig, die Versuche sowohl im Laser- als auch im MIG-Lötverfahren durchzuführen. Die Lötverfahren werden in der Praxis für unterschiedliche Anforderungen innerhalb der Karosserie eingesetzt. Das Fügen von hochfesten Strukturelementen oder Außenhautbauteilen erfordert in Abhängigkeit von den Anforderungen die Verwendung ausgewählter Zusatzwerkstoffe. Die Vielfältigkeit der Werkstoffe und der Anforderungen spiegelt sich in den Untersuchungen der vorliegenden Arbeit wieder. Für weitergehende Untersuchungen, speziell im hochfesten Blechbereich, soll die Arbeit entsprechende Grundlagen bieten. Als Vorlage für die Erarbeitung von experimentellen und theoretischen Methodiken der Prozessbetrachtung werden neben typischen Kupferloten neu entwickelte Lotlegierungen verwendet. Bei der Betrachtung der Kupferlegierungen werden die unterschiedlichen Einflüsse auf den Fügeprozess definiert und beschrieben. Es wird festgestellt, dass niedrig schmelzende Lote mit ausgewählter Legierungszusammensetzung im Gegensatz zu Eisenbasis-Schweißdrähten einen geringeren negativen Einfluss auf das Gefüge der Bleche im Nahtbereich ausüben. Um die thermische Beanspruchung, besonders in der Wärmeeinflusszone, während des Fügeprozesses gering zu halten, kann zusätzlich eine geeignete Stromquellentechnik zum Einsatz kommen. Mit Hilfe des „kalten“ Lichtbogens ist es möglich, die eingebrachte Streckenenergie weiter zu reduzieren. Faktoren, die den Energieeintrag beeinflussen, werden in der vorliegenden Arbeit in experimentellen und theoretischen Untersuchungen hinsichtlich ihrer Wirkung auf das Festigkeitsverhalten betrachtet. Es werden durch geeignete Legierungskombinationen die Einflüsse auf die Steigerung der Lötgeschwindigkeit und auf eine Verbesserung des Phosphatierungsverhaltens untersucht. Die Ergebnisse dieser Untersuchungen liefern die Informations- und Beweisbasis für die erarbeiteten Legierungssysteme und ermöglichen es, den optimierten Lötprozess an hochfesten Stahlblechen wissenschaftlich zu betrachten. Die Auswertung der wissenschaftlichen Experimente, dargestellt in den angefügten ausführlichen Tabellen, stellen die Zusammenhänge zwischen der Legierungsauswahl und der eingebrachten Streckenenergie dar. Die Erkenntnisse aus der vorliegenden Arbeit sollen für das Fügen von hochfesten Blechen die Entscheidung über die Auswahl geeigneter Zusatzwerkstoffe erleichtern. Die Ergebnisse der theoretischen Untersuchungen anhand mathematischer Modelle zur Beschreibung der physikalischen Prozesse der Wärmezufuhr durch Verwendung eines ausgewählten Lotes in Kombination mit geeigneter Stromquellentechnologie sind die Grundlage für die Optimierung des Lötprozesses. Die vorgeschlagenen Modelle zur Entwicklung und Optimierung von Lichtbogenlötprozessen mit neu entwickelten Lotlegierungen wurde im Rahmen der vorgelegten Arbeit an realen Blechqualitäten angewendet und überprüft. In den Ergebnissen hat sich bestätigt, dass die Verwendung spezieller Kupferlote zu verbesserten Verarbeitungseigenschaften führen, und damit Konzepte zum wirtschaftlich verbesserten Fügen angeboten werden.