Orientador: Marcelo dos Santos Pereira / Resumo: O desenvolvimento dos aços avançados de alta resistência tem sido a principal estratégia por parte das siderúrgicas para atenderem às demandas das indústrias automobilísticas por um material que permita a redução de peso, diminuição do consumo de combustível e, simultaneamente, que possibilite uma melhora na resistência ao impacto e na segurança dos passageiros. Neste contexto, o aço bifásico apresenta grande potencial tecnológico de aplicação, principalmente em componentes estruturais, em virtude de suas propriedades mecânicas especiais, como alta resistência à tração, alta taxa de encruamento e muito boa ductilidade. No entanto, é necessário minimizar os problemas dimensionais provocados pelo efeito do retorno elástico (springback) durante a estampagem, assim como, as falhas durante o processamento. Este trabalho tem por objetivo caracterizar um aço bifásico da classe 1000 MPa (DP980), quanto à microestrutura e propriedades mecânicas. A caracterização microestrutural, por meio de técnicas de microscopia óptica e eletrônica de varredura, determinaram as fases presentes de maneira qualitativa e quantitativa, assim como suas morfologias, com o objetivo de correlacionar a microestrutura com as propriedades mecânicas. Evidenciou-se a presença de grãos alongados, acompanhando o sentido de laminação da chapa e uma fração volumétrica de martensita de 51,4%. As características em relação aos processos de estampagem foram estudadas por meio do ensaio de dobramento para determinar o r... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The development of advanced high strength steels (AHSS) has been the main strategy for steelmakers to meet the demands of the automotive industry for a material that reduces weight and fuel consumption, provides shock resistance and vehicle safety. Within this context, dual-phase steel (DP) shows great technological potential, mainly in structural parts, due to its special mechanical properties, such as high tensile strength, work hardening, and ductility. However, it is necessary to minimize the dimensional problems caused by springback during stamping, along with the processing failures. This work aims to characterize a 1000 MPa (DP980) dual-phase steel regarding its microstructure and mechanical properties. The microstructural characterization, through optical microscopy and scanning electron microscopy, determined the present phases in a qualitative and quantitative manner, as well as their morphologies, in order to correlate microstructure with mechanical properties. The presence of elongated grains, following the direction of lamination of the sheet, as well as a volumetric fraction of martensite of 51.4% were observed. Features of the stamping processes were examined through a bending test to determine the springback in samples submitted to bending punches with radii of 5 mm and 0.3 mm. The samples tested with the 5 mm radius punch show a strong springback in function of the high strength of the steel. With the 0.3 mm radius punch, the springback decreases significantl... (Complete abstract click electronic access below) / Doutor

Este trabalho tem por objetivo avaliar os efeitos do hidrogênio em três diferentes tipos de aços de alta resistência mecânica. São descritos os fenômenos de introdução, difusão e aprisionamento de hidrogênio (H) dentro dos metais, em conjunto com os diferentes tipos de danos provocados devido à presença do hidrogênio. Os materiais de estudo são aços da família Advanced High Strength Steels (AHSS): aços Dual Phase (DP 1000 e DP 1200) e aço Martensítico (M 190). A introdução de hidrogênio nos materiais foi realizada através de carregamento catódico, o qual é representativo para as condições industriais a que se destinam. De modo a avaliar a influência do H nas propriedades mecânicas dos aços, os seguintes ensaios foram propostos: ensaio de carregamento com H, para determinar o conteúdo total de H (saturação) e conteúdo de H difusível (suscetibilidade a fragilização); ensaio de tração ao ar, para determinar a tensão no final da região elástica e resistência à tração na região do entalhe e ensaio de tração com carga constante em ambiente hidrogenado, para avaliar os efeitos provocados pela presença do H e determinar o patamar abaixo do qual o H não apresenta efeito crítico sobre o material. Os efeitos provocados pela aplicação de diferentes densidades de correntes (0,2 – 1,0 mA/cm²) foram avaliados nos ensaios de quantificação de H difusível. Conforme os resultados obtidos todos os aços apresentaram perdas na resistência mecânica à tração quando em ambiente hidrogenado, ou seja, sofreram fragilização por H. Os aços DP 1200 e M 190 (de microestrutura predominantemente martensítica) foram fortemente afetados, conforme evidenciado pela notável queda nos valores de tensão necessários para provocar a falha. Por outro lado, o aço DP 1000, de menor resistência mecânica, demonstrou menor suscetibilidade à fragilização, o que é atribuído a menor permeabilidade do H na microestrutura austenítica. / This work aims to evaluate the effects of hydrogen in three high- strength steel grades. The phenomena of hydrogen (H) entry, transport and trapping inside the metals, together with the different types of damages due to the presence of hydrogen are presented. The study materials are a range of AHSS steel grades: Dual Phase Steel (DP 1000 and DP 1200) and Martensitic Steel (M 190). The hydrogen entry was performed by cathodic charging, which is suitable for industrial applications. In order to evaluate the influence of H on the steel mechanical properties, the following tests were done: H charging, to measure total H content (saturation point) and diffusible H content (embrittlement susceptibility); uniaxial tensile test of uncharged samples to determine notched tensile strength values and the strength levels at the end of elastic region and constant load tensile testing carried out in hydrogen environment, to determine the threshold values where hydrogen has an effect on the material. DP 1200 and M 190 were strongly affected by H pre-charging, as shown by the significant drop in stress required to break them. On the other hand, DP 1000 showed a lower embrittlement susceptibility, which is attributed to its lower mechanical strength. The current densities effects (0.2 up to 1.0 mA/cm²) were evaluated during H charging to measure diffusible H content. All steels showed a drop in the tensile strength i.e. experienced hydrogen embrittlement. Steels with higher tensile strength, as DP 1200 and M 190, showed a much bigger drop that is related to the favorable characteristics of martensitic microstructure regarding to the hydrogen permeability and diffusivity.

Este trabalho tem por objetivo avaliar os efeitos do hidrogênio em três diferentes tipos de aços de alta resistência mecânica. São descritos os fenômenos de introdução, difusão e aprisionamento de hidrogênio (H) dentro dos metais, em conjunto com os diferentes tipos de danos provocados devido à presença do hidrogênio. Os materiais de estudo são aços da família Advanced High Strength Steels (AHSS): aços Dual Phase (DP 1000 e DP 1200) e aço Martensítico (M 190). A introdução de hidrogênio nos materiais foi realizada através de carregamento catódico, o qual é representativo para as condições industriais a que se destinam. De modo a avaliar a influência do H nas propriedades mecânicas dos aços, os seguintes ensaios foram propostos: ensaio de carregamento com H, para determinar o conteúdo total de H (saturação) e conteúdo de H difusível (suscetibilidade a fragilização); ensaio de tração ao ar, para determinar a tensão no final da região elástica e resistência à tração na região do entalhe e ensaio de tração com carga constante em ambiente hidrogenado, para avaliar os efeitos provocados pela presença do H e determinar o patamar abaixo do qual o H não apresenta efeito crítico sobre o material. Os efeitos provocados pela aplicação de diferentes densidades de correntes (0,2 – 1,0 mA/cm²) foram avaliados nos ensaios de quantificação de H difusível. Conforme os resultados obtidos todos os aços apresentaram perdas na resistência mecânica à tração quando em ambiente hidrogenado, ou seja, sofreram fragilização por H. Os aços DP 1200 e M 190 (de microestrutura predominantemente martensítica) foram fortemente afetados, conforme evidenciado pela notável queda nos valores de tensão necessários para provocar a falha. Por outro lado, o aço DP 1000, de menor resistência mecânica, demonstrou menor suscetibilidade à fragilização, o que é atribuído a menor permeabilidade do H na microestrutura austenítica. / This work aims to evaluate the effects of hydrogen in three high- strength steel grades. The phenomena of hydrogen (H) entry, transport and trapping inside the metals, together with the different types of damages due to the presence of hydrogen are presented. The study materials are a range of AHSS steel grades: Dual Phase Steel (DP 1000 and DP 1200) and Martensitic Steel (M 190). The hydrogen entry was performed by cathodic charging, which is suitable for industrial applications. In order to evaluate the influence of H on the steel mechanical properties, the following tests were done: H charging, to measure total H content (saturation point) and diffusible H content (embrittlement susceptibility); uniaxial tensile test of uncharged samples to determine notched tensile strength values and the strength levels at the end of elastic region and constant load tensile testing carried out in hydrogen environment, to determine the threshold values where hydrogen has an effect on the material. DP 1200 and M 190 were strongly affected by H pre-charging, as shown by the significant drop in stress required to break them. On the other hand, DP 1000 showed a lower embrittlement susceptibility, which is attributed to its lower mechanical strength. The current densities effects (0.2 up to 1.0 mA/cm²) were evaluated during H charging to measure diffusible H content. All steels showed a drop in the tensile strength i.e. experienced hydrogen embrittlement. Steels with higher tensile strength, as DP 1200 and M 190, showed a much bigger drop that is related to the favorable characteristics of martensitic microstructure regarding to the hydrogen permeability and diffusivity.

Este trabalho tem por objetivo avaliar os efeitos do hidrogênio em três diferentes tipos de aços de alta resistência mecânica. São descritos os fenômenos de introdução, difusão e aprisionamento de hidrogênio (H) dentro dos metais, em conjunto com os diferentes tipos de danos provocados devido à presença do hidrogênio. Os materiais de estudo são aços da família Advanced High Strength Steels (AHSS): aços Dual Phase (DP 1000 e DP 1200) e aço Martensítico (M 190). A introdução de hidrogênio nos materiais foi realizada através de carregamento catódico, o qual é representativo para as condições industriais a que se destinam. De modo a avaliar a influência do H nas propriedades mecânicas dos aços, os seguintes ensaios foram propostos: ensaio de carregamento com H, para determinar o conteúdo total de H (saturação) e conteúdo de H difusível (suscetibilidade a fragilização); ensaio de tração ao ar, para determinar a tensão no final da região elástica e resistência à tração na região do entalhe e ensaio de tração com carga constante em ambiente hidrogenado, para avaliar os efeitos provocados pela presença do H e determinar o patamar abaixo do qual o H não apresenta efeito crítico sobre o material. Os efeitos provocados pela aplicação de diferentes densidades de correntes (0,2 – 1,0 mA/cm²) foram avaliados nos ensaios de quantificação de H difusível. Conforme os resultados obtidos todos os aços apresentaram perdas na resistência mecânica à tração quando em ambiente hidrogenado, ou seja, sofreram fragilização por H. Os aços DP 1200 e M 190 (de microestrutura predominantemente martensítica) foram fortemente afetados, conforme evidenciado pela notável queda nos valores de tensão necessários para provocar a falha. Por outro lado, o aço DP 1000, de menor resistência mecânica, demonstrou menor suscetibilidade à fragilização, o que é atribuído a menor permeabilidade do H na microestrutura austenítica. / This work aims to evaluate the effects of hydrogen in three high- strength steel grades. The phenomena of hydrogen (H) entry, transport and trapping inside the metals, together with the different types of damages due to the presence of hydrogen are presented. The study materials are a range of AHSS steel grades: Dual Phase Steel (DP 1000 and DP 1200) and Martensitic Steel (M 190). The hydrogen entry was performed by cathodic charging, which is suitable for industrial applications. In order to evaluate the influence of H on the steel mechanical properties, the following tests were done: H charging, to measure total H content (saturation point) and diffusible H content (embrittlement susceptibility); uniaxial tensile test of uncharged samples to determine notched tensile strength values and the strength levels at the end of elastic region and constant load tensile testing carried out in hydrogen environment, to determine the threshold values where hydrogen has an effect on the material. DP 1200 and M 190 were strongly affected by H pre-charging, as shown by the significant drop in stress required to break them. On the other hand, DP 1000 showed a lower embrittlement susceptibility, which is attributed to its lower mechanical strength. The current densities effects (0.2 up to 1.0 mA/cm²) were evaluated during H charging to measure diffusible H content. All steels showed a drop in the tensile strength i.e. experienced hydrogen embrittlement. Steels with higher tensile strength, as DP 1200 and M 190, showed a much bigger drop that is related to the favorable characteristics of martensitic microstructure regarding to the hydrogen permeability and diffusivity.

In the pipe manufacturing and pipe processing industry, the demand for cost-effective pipes with high strength and good ductility is increasing. In the present study, the inductive longitudinal welding process was combined with a Quenching and Partitioning (Q&P) treatment to manufacture pipes with enhanced mechanical properties. The aim of the Q&P process is to establish a martensitic structure with increased retained austenite content. This allows for the beneficial use of both phases: the strength of martensite as well as the ductility of retained austenite. A 42SiCr steel, developed for Q&P processes, was joined at the longitudinal seam by a high-frequency induction (HFI) welding process and was subsequently heat-treated. The applied heat treatments included normalizing, austenitizing, quenching, and two Q&P strategies (Q&P-A/Q&P-B) with distinct quenching (Tq = 200/150 °C) and partitioning temperatures (Tp = 300/250 °C). Investigations of the microstructures revealed that Q&P tubes exhibit increased amounts of retained austenite in the martensitic matrix. Differences between the weld junction and the base material occurred, especially regarding the morphology of the martensite; the martensite found in the weld junction is finer and corresponds more to the lath-type morphology, compared to the base material in the circumference. In all zones of the welded tube circumference, retained austenite has been found in similar distributions. The mechanical testing of the individual tubes demonstrated that the Q&P treatments offer increased strength compared to all other states and significantly improved ductility compared to the quenched condition. Therefore, the approach of Q&P treatment of HFI-welded tubes represents a route for the mass production of high-strength tubular products with improved ductility.

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Efforts to reduce vehicle weight and improve crash performance have resulted in increased application of advanced high strength steels (AHSS) and a recent focus on the weldability of these alloys. Resistance spot welding (RSW) is the primary sheet metal welding process in the manufacture of automotive assemblies. Integration of AHSS into the automotive architecture has brought renewed challenges for achieving acceptable welds. The varying alloying content and processing techniques has further complicated this initiative. The current study examines resistance spot welding of high strength and advance high strength steels including high strength low alloy (HSLA), dual phase (DP) and a ferritic-bainitic steel (590R). The mechanical properties and microstructure of these RSW welded steel alloys are detailed. Furthermore a relationship between chemistries and hardness is produced. The effect of strain rate on the joint strength and failure mode is also an important consideration in the design of welded structures. Current literature, however, does not explain the effects of weld microstructure and there are no comprehensive comparisons of steels. This work details the relationship between the joint microstructure and impact performance of spot welded AHSS. Quasi-static and impact tests were conducted using a universal tensile tester and an instrumented drop tower, respectively. Results for elongation, failure load and energy absorption for each material are presented. Failure modes are detailed by observing weld fracture surfaces. In addition, cross-sections of partially fractured weldments were examined to detail fracture paths during static loading. Correlations between the fracture path and mechanical properties are developed using observed microstructures in the fusion zone and heat-affected-zone. Friction stir spot welding (FSSW) has proven to be a potential candidate for spot welding AHSS. A comparative study of RSW and FSSW on spot welding AHSS has also been completed. The objective of this work is to compare the microstructure and mechanical properties of Zn-coated DP600 AHSS (1.2mm thick) spot welds conducted using both processes. This was accomplished by examining the metallurgical cross-sections and local hardnesses of various spot weld regions. High speed data acquisition was also used to monitor process parameters and attain energy outputs for each process.

Efforts to reduce vehicle weight and improve crash performance have resulted in increased application of advanced high strength steels (AHSS) and a recent focus on the weldability of these alloys. Resistance spot welding (RSW) is the primary sheet metal welding process in the manufacture of automotive assemblies. Integration of AHSS into the automotive architecture has brought renewed challenges for achieving acceptable welds. The varying alloying content and processing techniques has further complicated this initiative. The current study examines resistance spot welding of high strength and advance high strength steels including high strength low alloy (HSLA), dual phase (DP) and a ferritic-bainitic steel (590R). The mechanical properties and microstructure of these RSW welded steel alloys are detailed. Furthermore a relationship between chemistries and hardness is produced. The effect of strain rate on the joint strength and failure mode is also an important consideration in the design of welded structures. Current literature, however, does not explain the effects of weld microstructure and there are no comprehensive comparisons of steels. This work details the relationship between the joint microstructure and impact performance of spot welded AHSS. Quasi-static and impact tests were conducted using a universal tensile tester and an instrumented drop tower, respectively. Results for elongation, failure load and energy absorption for each material are presented. Failure modes are detailed by observing weld fracture surfaces. In addition, cross-sections of partially fractured weldments were examined to detail fracture paths during static loading. Correlations between the fracture path and mechanical properties are developed using observed microstructures in the fusion zone and heat-affected-zone. Friction stir spot welding (FSSW) has proven to be a potential candidate for spot welding AHSS. A comparative study of RSW and FSSW on spot welding AHSS has also been completed. The objective of this work is to compare the microstructure and mechanical properties of Zn-coated DP600 AHSS (1.2mm thick) spot welds conducted using both processes. This was accomplished by examining the metallurgical cross-sections and local hardnesses of various spot weld regions. High speed data acquisition was also used to monitor process parameters and attain energy outputs for each process.

No description available.