No description available.

Eine effiziente und effektive Technologieentwicklung und –optimierungen im Bereich der Umform- und Fertigungstechnik erfolgt heutzutage fast ausschließlich rechnergestützt auf Basis der Finiten Elemente Methode (FEM) oder der Finiten Differenzen Methode (FDM). Die aktuellen Umformsimulationssysteme sind in der Lage die notwendige Energie der Anlagen, den prozessbedingten Stofffluss des Umformgutes inkl. der resultierenden Temperaturen und die Spannungen des Halbzeuges bzw. Bauteiles als auch der Werkzeuge vorauszuberechnen. Allerdingssind bereits dafür die sehr sensitiven Materialdaten, wie z. B. temperatur- und umformgeschwindigkeitsabhängige Fließkurven, Wärmeleitfähigkeit usw., notwendig. Momentane Forschungsaktivitäten beschäftigen sich damit, den nächsten Schritt der FE-Simulation zu bewältigen, in dem die Gefügeentwicklung und die daraus resultierenden mechanischen Eigenschaften (Zugfestigkeit, Bruchdehnung etc.) numerisch ermittelt werden können. Auch dafür müssen sehr aufwendige und materialspezifische Materialdaten generiert und modelliert und abschließend in Simulationssysteme über Schnittstellen implementiert werden. Die Vorhersage zu Verschleiß und Versagen von Werkzeugen wird in Zukunft immer mehr in das Interesse von Anwendern von FE-Software rücken, um die kompletten Einflussgrößen der Prozesse abzubilden. Dieser Beitrag soll am Beispiel einer aushärtbaren Aluminiumlegierung (EN AW 6.xxx) verdeutlichen, wie unterschiedlich das Materialverhalten in Abhängigkeit des Ausgangszustandes (stranggepresst, stranggegossen) und der Erwärmungsmodi (konvektiv, induktiv) sein kann und wie sich diese Variationen für ein und denselben Werkstoff auf die Berechnungsgenauigkeit ausgewählter Warmmassivumformprozesse (Reckwalzen, Gesenkschmieden) auswirkt.

Novos tratamentos térmicos e a otimização dos processos de conformação têm contribuído para o desenvolvimento de microestruturas multifásicas com excelente combinação de ductilidade e resistência mecânica. Parte dessa melhoria depende da presença de austenita retida, de sua estabilidade e fração volumétrica. O presente trabalho tem como objetivo caracterizar a evolução da microestrutura e comportamento das propriedades mecânicas do aço dual phase classe 600 (DP 600), após tratamentos térmicos intercríticos de têmpera e partição (Q&P) e reversão da martensita, assim como tratamentos termomecânicos de simulação física da estampagem a quente (HS), variando a deformação em 10% (HS 10) e 30% (HS 30), e combinando estampagem a quente com subsequente tratamento de têmpera e partição (HSQ&P). Duas condições microestruturais de partida diferentes foram utilizadas nos tratamentos térmicos. Para os tratamentos térmicos e termomecânicos Q&P, HS e HSQ&P a microestrutura de partida foi a bifásica (ferrita e martensita). Já para o tratamento térmico de reversão a microestrutura de partida foi modificada para martensítica. Os tratamentos puramente térmicos foram realizados no dilatômetro Bähr do Laboratório de Transformações de Fase (LTF); entretanto, os tratamentos termomecânicos foram feitos no simulador termomecânico Gleeble®, acoplado à linha de difração de raios X (XTMS) do Laboratório Nacional de Nanotecnologia (LNNano). A análise microestrutural foi feita com suporte de microscopia ótica (MO) e eletrônica de varredura (MEV-FEG), EBSD, e difração de raios X in situ e convencional. Avaliaram-se as propriedades mecânicas por ensaio de tração em corpos de prova sub-size e endentação instrumentada. As amostras Q&P, HS e HSQ&P foram submetidas a ensaios exploratórios de resistência ao trincamento por hidrogênio (HIC) segundo a norma NACE TM0284. Adicionalmente, foi feita a medição de hidrogênio ancorado na microestrutura estudada, após tratamentos, utilizando a técnica de dessorção térmica disponível no LNNano. A avaliação das mudanças microestruturais e de propriedades mecânicas após tratamentos térmicos foram discutidas separadamente para cada microestrutura de partida. Os resultados dos processos Q&P, HS e HSQ&P no aço, mostraram que a evolução da microestrutura levou a formação de uma microestrutura mais complexa do que a microestrutura ferrítico-martensítica simples do material como recebido. A complexa microestrutura é dada pela formação de ferrita epitaxial durante a etapa de tratamento intercrítico, ferrita induzida por deformação (DIFT) na etapa de deformação em alta temperatura e bainita na etapa de partição. Essa mistura microestrutural levou a variações na relação das frações volumétricas de ferrita e martensita em relação às frações iniciais do aço, assim como na presença de austenita retida e sua estabilidade. Com base nos resultados é possível afirmar que o processo Q&P produz um aumento nas propriedades mecânicas do material. Por outro lado, após o ensaio de HIC todas as amostras apresentaram susceptibilidade ao trincamento; contudo, a severidade do dano foi maior nas amostras deformadas HS 30. Os ensaios preliminares de dessorção mostraram maior aprisionamento de hidrogênio em armadilhas reversíveis nas amostras HSQ&P e irreversíveis na amostra HS 30. Na segunda parte, os resultados do tratamento de reversão sugerem que, em geral, a microestrutura do aço processado compreende uma morfologia em ripas de ferrita intercrítica, martensita e filmes de austenita retida. A maior temperatura de reversão intercrítica resultou em menor fração de ferrita intercrítica. Por outro lado, a temperatura intercrítica de reversão influenciou significativamente a estabilidade da austenita retida. Uma alta fração de austenita retida foi obtida a uma temperatura ligeiramente acima da temperatura Ac1. Um segundo ciclo de reversão promoveu a difusão de C e Mn para a austenita revertida tornando-a mais estável a temperatura ambiente. / Novel Heat Treatments and the optimization of the forming processes have contributed to the development of multiphase microstructures with attractive combinations of ductility and mechanical resistance. This improvement partially depends on the presence, stability and volume fraction of retained austenite. The objective of this work is to characterize the evolution of the microstructure and mechanical properties of a class 600 dual phase steel (DP 600), as a function of the thermal and thermomechanical history. Two initial microstructures were used in this study. A ferritic-martensitic microstructure was used as the starting condition for inter-critical heat treatments followed by quenching and partitioning (Q&P) and for the thermomechanical simulations of the hot stamping (HS) process. The latter applying deformations of 10% (HS 10) and 30% (HS 30) combining hot stamping with subsequent quenching and partition (HSQ&P). The thermal cycles were performed in a Bähr dilatometer at the Laboratory of Phase Transformations (LTF), then duplicated using a Gleeble® thermomechanical simulator, coupled to the X-ray Scattering and Thermo-mechanical Simulation beamline (XTMS) at the Brazilian Nanotechnology National Laboratory (LNNano). The microstructural analysis was performed using optical microscopy (MO) and scanning electron (SEM-FEG), Electron Backscatter Diffraction (EBSD), and in situ and conventional X-ray diffraction. The mechanical properties were evaluated by tensile testing on sub-size specimens and by instrumented macro-nano indentation tests. The evolution of the microstructure and mechanical properties for each starting microstructure was discussed separately. The Q&P, HS and HSQ&P samples were submitted to exploratory tests of resistance to hydrogen induced cracking (HIC) according to NACE TM0284. Additionally, hydrogen measurements were performed for the microstructures obtained after Q&P and HDQ&P using the thermal desorption technique at LNNano. After Q&P, HS and HSQ&P, the resultant microstructure was more complex than the as-received ferritic-martensitic condition. Such complexity comes from the formation of epitaxial ferrite from the former ferritic phase during the intercritical treatment step, the deformation induced ferrite (DIFT) and the bainite formation during the partitioning step. This led to variations in the volumetric fraction of ferrite and martensite in relation to the initial fractions of the as-received condition, as well as the presence of retained austenite and its stability upon cooling. The Q&P process increased the mechanical properties of the material. On the other hand, all microstructures showed susceptibility to hydrogen cracking after 72 hours of H2S exposure tests. However, the damage was more severe for the HS samples with 30% of deformation. The preliminary desorption tests showed greater hydrogen trapping in reversible traps after HSQ&P and in irreversible traps for the HS with 30% deformation. A second set of experiments was conducted for a different microstructure consisting of a fully martensitic matrix as the initial condition. After intercritical reversion, the resultant microstructure comprised intercritical lath-like ferrite, martensite laths and retained austenite films. The higher the intercritical reversion temperature, the smaller the fraction of intercritical ferrite. On the other hand, the transformation temperature significantly influenced the stability of the retained austenite. The highest fraction of retained austenite was obtained when the transformation occurred slightly above the Ac1 temperature. A double intercritical reversion cycle promoted the diffusion of C and Mn to the reversed austenite making it more stable upon cooling to room temperature, leading to a better combination of strength and ductility.

In an automotive crash event, hot stamped, die quenched martensitic structural components have been shown to provide excellent intrusion resistance. These alloys exhibit only limited ductility, however, which may limit the overall impact performance of the component. The introduction of lower strength and more ductile “tailored” properties within some regions of a hot stamped component has the potential to improve impact performance. One approach being applied to achieving such tailored properties is through locally controlling the cooling rate within the stamping die. The primary motivation for the current work is to understand the role of cooling rate on the as-quenched mechanical response of tailored hot stampings, which has required characterization of the high strain rate mechanical behaviour of tailored hot stamped boron steel. The effect of cooling rate and resulting microstructure on the as-quenched mechanical behavior of USIBOR® 1500P boron steel at strain rates between 10-3 and 103 s-1 was investigated. Specimens quenched at rates above the critical cooling rate (~27 °C/s) exhibited a fully martensitic microstructure with a UTS of ~1,450 MPa. Sub-critical cooling rates, in the range 14°C/s to 50 °C/s, resulted in as-quenched microstructures ranging between bainitic to martensitic, respectively. Tension tests revealed that predominantly bainitic material conditions (14 °C/s cooling rate) exhibited a lower UTS of 816 MPa compared to 1,447 MPa for the fully martensitic material condition (50 °C/s cooling rate) with a corresponding increase in elongation from 0.10 to 0.15 for the bainitic condition. The reduction in area was 70% for the bainitic material condition and 58% for the martensitic material conditions which implied that a tailored region consisting of bainite may be a desirable candidate for implementation within a hot stamped component. The strain rate sensitivity was shown to be moderate for all of the as-quenched material conditions and the measured flow stress curves were used to develop a strain rate sensitive constitutive model, the “Tailored Crash Model (TCM)”. The TCM accurately reproduced the measured flow stress curves as a function of effective plastic strain, strain rate and Vickers hardness (or area fraction of martensite and bainite). The effect of deformation during quenching and the associated shift in the CCT diagram on the subsequent constitutive response was also examined for this material. Specimens were simultaneously quenched and deformed at various cooling rates to achieve a range of as-quenched microstructures that included ferrite in addition to martensite and bainite. Tensile tests conducted on these specimens at strain rates ranging from 0.003 s-1 to ~80 s-1 revealed that the presence of ferrite resulted in an increase in uniform elongation and n-value which increased overall energy absorption for a given hardness level. The strain rate sensitivity was shown to be moderate for all of the as-quenched material conditions and the TCM constitutive model was extended to account for the presence of ferrite. This extended constitutive model, the “Tailored Crash Model II (TCM II)”, has been shown to predict flow stress as a function of effective plastic strain, strain rate and area fraction of martensite, bainite and ferrite. As a validation exercise, uniaxial tension test simulations of specimens extracted from the transition zone of a hot stamped lab-scale B-pillar with tailored properties [1] were performed. The measured hardness distribution along the gauge length of the tensile specimens was used as input for the TCM constitutive model to define the element constitutive response used in the finite element (FE) models. The measured stress versus strain response and strain distribution during loading (measured using digital image correlation) was in excellent agreement with the FE models and thus validated the TCM constitutive model developed in this work. Validation of the TCM II version of the model is left for future work.

In an automotive crash event, hot stamped, die quenched martensitic structural components have been shown to provide excellent intrusion resistance. These alloys exhibit only limited ductility, however, which may limit the overall impact performance of the component. The introduction of lower strength and more ductile “tailored” properties within some regions of a hot stamped component has the potential to improve impact performance. One approach being applied to achieving such tailored properties is through locally controlling the cooling rate within the stamping die. The primary motivation for the current work is to understand the role of cooling rate on the as-quenched mechanical response of tailored hot stampings, which has required characterization of the high strain rate mechanical behaviour of tailored hot stamped boron steel. The effect of cooling rate and resulting microstructure on the as-quenched mechanical behavior of USIBOR® 1500P boron steel at strain rates between 10-3 and 103 s-1 was investigated. Specimens quenched at rates above the critical cooling rate (~27 °C/s) exhibited a fully martensitic microstructure with a UTS of ~1,450 MPa. Sub-critical cooling rates, in the range 14°C/s to 50 °C/s, resulted in as-quenched microstructures ranging between bainitic to martensitic, respectively. Tension tests revealed that predominantly bainitic material conditions (14 °C/s cooling rate) exhibited a lower UTS of 816 MPa compared to 1,447 MPa for the fully martensitic material condition (50 °C/s cooling rate) with a corresponding increase in elongation from 0.10 to 0.15 for the bainitic condition. The reduction in area was 70% for the bainitic material condition and 58% for the martensitic material conditions which implied that a tailored region consisting of bainite may be a desirable candidate for implementation within a hot stamped component. The strain rate sensitivity was shown to be moderate for all of the as-quenched material conditions and the measured flow stress curves were used to develop a strain rate sensitive constitutive model, the “Tailored Crash Model (TCM)”. The TCM accurately reproduced the measured flow stress curves as a function of effective plastic strain, strain rate and Vickers hardness (or area fraction of martensite and bainite). The effect of deformation during quenching and the associated shift in the CCT diagram on the subsequent constitutive response was also examined for this material. Specimens were simultaneously quenched and deformed at various cooling rates to achieve a range of as-quenched microstructures that included ferrite in addition to martensite and bainite. Tensile tests conducted on these specimens at strain rates ranging from 0.003 s-1 to ~80 s-1 revealed that the presence of ferrite resulted in an increase in uniform elongation and n-value which increased overall energy absorption for a given hardness level. The strain rate sensitivity was shown to be moderate for all of the as-quenched material conditions and the TCM constitutive model was extended to account for the presence of ferrite. This extended constitutive model, the “Tailored Crash Model II (TCM II)”, has been shown to predict flow stress as a function of effective plastic strain, strain rate and area fraction of martensite, bainite and ferrite. As a validation exercise, uniaxial tension test simulations of specimens extracted from the transition zone of a hot stamped lab-scale B-pillar with tailored properties [1] were performed. The measured hardness distribution along the gauge length of the tensile specimens was used as input for the TCM constitutive model to define the element constitutive response used in the finite element (FE) models. The measured stress versus strain response and strain distribution during loading (measured using digital image correlation) was in excellent agreement with the FE models and thus validated the TCM constitutive model developed in this work. Validation of the TCM II version of the model is left for future work.

This report evaluates the influence of the die material on the cooling rate and martensitic transformation of press hardened sheets. The goal was to increase the thicknesses of sheets that can form fully martensitic microstructure when press hardened. To achieve this, a numerical- and an experimental method was used and results were compared to assess the impact of die material change. The tests were conducted with two die materials, a ductile cast iron according to standard EN‑GJS‑700‑2 and a casted steel according to standard EN 1.6220. Two sheet materials, Hardox400 and Hardox450, were press hardened and two different thicknesses were evaluated. Simulations have been designed with temperature dependent material properties based on data gathered from the literature survey. All simulations indicated an improved cooling rate over the entire temperature spectrum when changing from the iron die to the steel die.   An experimental procedure has been performed using two different dies, both planar and of approximately the same thickness. Thermocouples were used to obtain cooling curves of all sheets during quenching. Samples were taken from each sheet and the hardness, microstructure and the present phases were investigated.   The experiments concluded that the thinner sheets, when quenched, experienced an overall increase in cooling rate in the steel die compared with the iron die. A total reduction in cooling time by 37.5%-43,7% was observed over the entire temperature span. However, only the Hardox400 sheet fully formed martensite, as the cooling of the Hardox450 sheets still was not fast enough in either of the dies. For the thicker sheets, the experiments also indicated a reduction in total cooling time. The total cooling time was reduced by 23% when pressed in the steel die compared to the iron die. This improvement, however, was not observed at higher temperatures. At the critical temperature span between 800˚C and 500˚C, the sheet showed no improvements in cooling rate with the die material change. Both the hardness measurements and the microstructure evaluation of the thicker sheets indicated a pearlitic-martensitic microstructure. As both the simulations and experiments indicated similar improvements, the increase in cooling rate could be accredited to the die material change.  This concludes that the heat transfer properties of the die material affected the cooling characteristics of the process. It was also concluded that the thinner sheets experienced a reduced cooling time over the entire temperature spectrum with the die material change. The thicker sheet, however, only experienced a reduced cooling time in the lower temperature span. Thus, changing the die material did not affect the hardening of the thicker sheets. This ultimately resulted in an unsuccessful attempt to increase the possible thickness of sheets with fully martensite microstructure. The improvements observed for the thinner sheets, are however promising and could be further evaluated for another sheet material. / I denna avhandling har relationen mellan ett verktygmaterials kylningseffekt och genomhärdningsförmåga under presshärdning utvärderats, med målet att öka tjockleken på plåtmaterial som genomhärdar. För att åstadkomma detta har en numerisk undersökning och en experimentell undersökning utförts för att bedöma effekten av ett byte av verktygsmaterial. Undersökningarna utfördes med två verktygsmaterial, ett segt gjutjärn enligt standard EN‑GJS‑700‑2 och ett gjutet stål enligt standard EN 1.6220. Under testerna användes även två olika plåtmaterial, Hardox400 och Hardox450, i två olika tjocklekar. Simuleringen har utformats med relevanta temperaturberoende materialparametrar erhållna från teorin. Resultaten från simuleringarna påvisade att ett byte av verktygsmaterial gav snabbare kylning av all plåtar över hela det undersökta temperaturspannet. Den experimentella undersökningen har utförts med två olika pressverktyg, båda plana med likartad tjocklek. Temperaturgivare användes för att erhålla kylkurvor för plåtarna när de kyldes i verktygen. Hårdhet, mikrostruktur och erhållna faser undersöktes för varje plåt. Experimenten påvisade att de tunnare plåtarna kyldes snabbare i stålverktyget än i järnverktyget över hela temperaturspannet, och resulterade i 37,5%-43,7% kortare kyltider. Trots detta, var det endast Hardox400-plåten som genomhärdades, då kylningen av Hardox450-plåten inte var tillräckligt snabb i något av verktygen. Även de tjockare plåtarna uppvisade en totalt kortare kyltid. Kyltiden minskade med 23% i stålverktyget jämfört med järnverktyget. Denna förbättring observerades dock inte över hela temperaturspannet. Vid höga temperaturer kylde stålverktyget långsammare än järnverktyget. De släckta plåtarna erhöll en perlitisk/martensitisk struktur vid komplett svalning, oberoende av verktygsmaterial. Eftersom både simuleringarna och experimenten påvisade liknande förbättringar, kan den ökade kylhastigheten härledas till bytet av verktygsmaterial. Detta fastställer att värmetransportegenskaperna hos verktygsmaterialet hade en inverkan på kylningen i processen. Studien påvisade också att de tunnare plåtarna kyldes fortare över hela temperaturspannet vid materialbytet. De tjockare plåtarna uppvisade dock endast en kortare kyltid i det lägre temperaturspannet. Därmed förblev härdningen av de tjockare plåtarna opåverkad av materialbytet. Detta resulterade i att presshärdning av ökad plåttjocklek för det undersökta materialet inte lyckades. De observerade förbättringarna hos de tunnare plåtarna är dock mycket lovande och bör studeras vidare.

In recent years, the automotive industry has turned to press hardened steels (PHS) to improve passenger safety while enabling vehicle weight reduction. To form the complex shapes required for this purpose, they are often direct hot press formed. It is possible to provide corrosion resistance to these parts by galvanizing the PHS sheets prior to direct hot press forming (DHPF). However, the austenitization of the galvanized steel causes the Zn-based coating to transform into two intermetallic phases. These are iron-rich α-Fe(Zn) and zinc-rich Г-Fe3Zn10. The Г-Fe3Zn10 is liquid during traditional DHPF, and the applied stress can result in liquid metal embrittlement (LME). Recently, two new grades of PHS have been developed, which allow for DHPF at 600-700°C, below the Fe-Zn peritectic temperature at 782°C, thus avoiding LME. These prototype PHS grades are designated 2%Mn (0.2C-2Mn-0.25Si-0.005B (wt%)) and 2.5%Mn (0.2C-2.5Mn-0.25Si-0.005B (wt%)). The objective of this work is to determine the effect of DHPF on the ability of a Zn-based coating to provide robust cathodic protection to the two prototype PHS. Galvanized panels of both the 2%Mn and 2.5%Mn steel were DHPF with a U-shape die at 700°C. The surface and cross-section of the coating were examined to determine the effects of DHPF on the coating surface. Die friction during DHPF resulted in die wiping on the wall of the part, leading to removal of surface Г-Fe3Zn10. In cross-section, coating cracks were present at the wall and corner of the U-shape part due to the deformation during DHPF. Potentiodynamic polarization scans were used to determine the corrosion potential of the coating, and this was used to calculate the driving force for cathodic protection using the difference in corrosion potential between the coating phases and the substrate. It was found that only Γ-Fe3Zn10 provided robust cathodic protection to both steel substrates, and the driving force for cathodic protection was lower for the coated DHPF 2.5%Mn steel. Galvanostatic scans were used to evaluate dissolution kinetics of coating phases. Robust cathodic protection was provided by the galvanized coating for austenitization times of 30 - 120 s for the 2%Mn substrate and 30 - 60 s for the 2.5%Mn substrate. The duration that robust cathodic protection was provided was shortest at the wall of the U-shape part. This result was attributed to die wiping caused by DHPF, where the surface is smoothed by die friction. When there is less Г-Fe3Zn10 in the coating, such as at longer austenization times, surface Г-Fe3Zn10 was removed and an increased amount of α-Fe(Zn) is exposed, which does not provide robust cathodic protection. In addition, coating cracks form along α-Fe(Zn) grain boundaries after austenitization for 180 s on all examined regions of the U-shape part, allowing a greater surface area of the coating exposed to electrolyte, further increasing dissolution of the coating. / Thesis / Master of Applied Science (MASc)

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Despite the high demand for press-hardenable steel (PHS) with coatings that provide sacrificial cathodic protection, Zn-based coatings have experienced limited use due to the significant challenges associated with avoiding liquid metal embrittlement (LME) while maintaining robust cathodic protection when using conventional PHS materials and processing techniques. The present research addresses these challenges by reducing the conventional direct hot press forming (DHPF) temperature to between 600–700 °C, such that forming and quenching occurs well below the Zn(Fe)liq → Г-Fe3Zn10 peritectic temperature of 782 °C, thereby removing the conditions necessary for LME to occur while allowing for formation of the cathodically-protective Г-Fe3Zn10 phase. The objective of this work was to define a process window for two galvanized prototype PHS alloys with compositions of 0.20C-2.01Mn-0.26Si-0.005B and 0.19C-2.5Mn-0.26Si-0.005B (wt%) that would result in fully martensitic microstructures, tensile strengths (TS) ≥ 1500 MPa, and robust cathodic protection, defined as attaining ≥ 15 vol% Г-Fe3Zn10 in the coating microstructure, while avoiding LME. Accomplishing this task involved characterizing both grades as a function of austenization time, stamping temperature, and strain imposed by the forming process in order to define process windows that resulted in parts that met the aforementioned property requirements. It was found that the approach of increasing the Mn content relative to conventional PHS grades was successful in improving the hardenability sufficiently to enable the formation of fully martensitic microstructures despite the lower effective cooling rates associated with the reduced DPHF temperatures. Microstructural imaging and tensile testing demonstrated that, for both prototype PHS grades, a process window exists for the production of parts that satisfy the targets of the formation of fully martensitic microstructures and TS ≥ 1500 MPa while exhibiting uniform elongation of about 0.05 followed by significant post-uniform elongation. The effect of DHPF temperature and strain imposed by the forming process on mechanical properties was found to be negligible. Tensile tests and fractography revealed that reducing the DHPF temperature to between 600–700 °C was successful in preventing LME, thereby allowing samples to fracture in a ductile manner. Micro-cracking in the coating of the DHPF part was observed; however, these cracks were arrested at the coating-substrate interface. For all tested conditions, the coating met the target of ≥ 15 vol% Г-Fe3Zn10, implying that robust cathodic protection is expected. Based on the results of these experiments, it was concluded that DHPF process windows that meet all of the property targets include austenization times and DHPF temperatures of 120–180 s and 600–700 °C, respectively, for the 2Mn grade, and 60–180 s and 600–700 °C, respectively, for the 2.5Mn grade. / Thesis / Master of Science (MSc)

O desenvolvimento de métodos de simulação física e numérica tem criado novas possibilidades de otimização dos processos relacionados à estampagem com inclusão de processos industriais reais. Portanto, recorrendo à aplicação destes métodos de análise, é possível avaliar a transformação mecânica e as transformações de fase que ocorrem no material e prever as interações entre as propriedades dos materiais no processo de conformação, o comportamento constitutivo do material, as variáveis de otimização do processo, bem como a previsão das tensões e deformações a fim de estabelecer a melhor relação material-processo-desempenho. A introdução e crescente utilização de aços avançados de alta resistência (AHSS) em aplicações automotivas exige uma maior compreensão dos fenômenos físicos envolvidos no processamento termomecânico a fim de otimizar a performance da peça final fabricada. O presente trabalho teve como objetivo avaliar experimentalmente o processo de estampagem a quente, com posterior tratamento térmico de têmpera e partição e analisar as microestruturas formadas e suas propriedades mecânicas. A formação de microestruturas durante o processo de estampagem a quente e de têmpera e partição foi avaliada neste trabalho por simulação física em simulador termomecânico Gleeble, acoplado à uma linha de difração de raios X (XTMS) de feixe de luz síncrotron no Laboratório Nacional de Nanotecnologia (LNNano). Foram avaliadas a partição do carbono, a estabilidade térmica da austenita retida e a formação de microconstituintes resultantes da transformação da austenita durante resfriamento forçado (têmpera), seguido de partição de carbono em patamares isotérmicos. Foram utilizadas técnicas de caracterização com apoio de microscopia eletrônica (MEV-FEG e STEM), EBSD, tomografia de sonda atômica (APT) e avaliação de propriedades mecânicas por ensaios de tração e indentação instrumentada. A análise numérica foi realizada por meio do método dos elementos finitos (MEF) e por elementos finitos orientada a objetos (OOF, Object Oriented Finite Element Analysis) visando estabelecer correlações entre microestrutura e propriedades mecânicas, comparando com resultados experimentais. Os resultados e conclusões obtidos no projeto, além de possibilitarem a identificação dos mecanismos fundamentais de geração de microestruturas durante o processo, auxiliam no projeto de aços AHSS estampados a quente, usados principalmente na indústria automobilística, na busca pela redução do consumo de combustível, através da redução do peso, e pelo aumento da segurança dos passageiros. / The development of numerical and physical simulation methods has created new possibilities regarding the optimization of metal forming processes, taking into account real industrial forming processes. Therefore, by applying such methods of analysis it is now possible to assess the material phase transformations and predict the interactions between material properties and the forming process, the constitutive behavior of the material, and optimize process variables as well as predicting the best material-process-performance relationship. The increasing usage of Advanced High Strength Steels (AHSS) in automotive applications demands a better insight of the physical phenomena involved in the thermomechanical processing in order to optimize the performance of the final manufactured part. Thermomechanical simulation of the hot stamping, quenching and partitioning process was carried out in a Gleeble machine coupled to the XTMS Synchrotron X-ray diffraction line at the National Nanotechnology Laboratory (LNNano). Carbon partitioning, carbon contents, and amount of retained austenite, martensite, bainite and ferrite was assessed online during the experiments. In addition, characterization techniques by optical, electron microscopy (FEG-SEM and STEM), EBSD, and Atom Probe Tomography (APT) were applied. Mechanical testing of subsize specimens of the processed steels was performed by means of tensile tests and macro and nanoindentation tests. The numerical analysis was performed using the finite element method (FEM) and object-oriented finite element technique (OOF). The results were compared with the experimental results of mechanical testing of specimens used in the thermomechanical simulations and with hot stamped sheets, where quenching and partitioning were carried out. The results and conclusions obtained in this project allow the identification of the fundamental mechanisms of the process, helping the design of the hot stamping process for AHSS steels used primarily in the automotive industry, seeking weight reduction to improve fuel economy and increased passenger safety.