The IF steel was cycled at strain amplitude of £`max = 0.3% and the samples were cycled at 3000¡B6000 and 13000 for observation of surface crack, respectively. The microstructure of the surface and interior of IF steel were examined by the SEM under BEI/ECCI mode. At 3000 cycles, we can find that many persistent slip bands (PSBs) form in grains, but it¡¦s not easy to find cracks. The main microstructures in the interior are dislocation loop patches and dislocation walls. The microstructures at the surface are dislocation cells, and we find the dislocation cells arrange along their prefer orientation in some grains. At 6000 cycles, there are many cracks on the surface, but the depth of cracks may be less than 10£gm. The mainly microstructures in the interior and on the surface are almost the same as those at 3000 cycles, but very few dislocation cells can be observed in the interior. When the fatigue cycles increase to 13000 cycles, there are many long cracks on the surface. The microstructure of crack tips are misorientation cells and the size of misorientation cells are less than 2£gm. It means that the length of cracks will develop if the fatigue cycles increase. As the result of the observation of surface cracks longer than 50 um in samples at 6000 and 13000 cycles, the microstructure of crack tips consists of misorientation cells which imply a propagating crack.

none

The overall aim of the project is to examine the rivetability of new steels and to investigate the mechanical behaviour of self-piercing riveted (SPR) aluminium-steel hybrid structures for automotive applications. Interstitial Free Steel (I.F.) of 1.2 mm thickness was joined to Aluminium 5754 of 2 mm thickness and Aluminium 5182 (coated and uncoated) of 1.5 mm thickness. The work began by initially conducting a quality assessment of the various joints that were produced in order to establish the optimum conditions for joining the various sample combinations to be investigated. A relationship was established between the head height and the interlock distance on the one hand and between the interlock distance and the lap shear strength of samples. It was also established that for higher lap shear strength, it is preferable to use the stronger material (I.F. steel) as the pierced sheet and the weaker material (5182) as the locked sheet. However, the results showed that this rule could not be applied for predicting the fatigue behaviour of SPR joints between I.F. steel and 5182. An investigation of the fatigue failure mechanisms was undertaken and possible reasons for this behaviour are discussed. The influence of fretting was also investigated by using scanning electron microscopy and reported. The fatigue behaviour of Dual Phase (DP600 + 5182) SPR joints was investigated. It was observed that the position of fatigue crack initiation differed with the maximum applied load. An explanation for this observation was provided by considering the failure mechanism of the samples under different load levels. The study also showed how fretting led to the initiation of fatigue cracks. The corrosion behaviour of (I.F. steel + 5182) samples was investigated by conducting tests in a salt spray according to the ASTM B117-97 standard. Three types of corrosion were observed; galvanic corrosion, differential aeration corrosion, uniform corrosion and are discussed. The weight change with time was monitored and was used to describe the corrosion behaviour. The lap shear strength was measured as a function of corrosion time. The presence of the corrosion product within the overlap was observed to greatly influence the lap shear strength behaviour. A further study was carried out in order to examine the influence of the individual alloys on the corrosion of the SPR samples. In this part the potential influence of pulse current treatment on corrosion was also investigated and was observed to increase greatly the corrosion resistance of the I.F. steel. Principal findings for this observation are also provided.

Os aços livres de intersticiais (IF) têm sido usados em todo o mundo de um modo crescente desde a década de 70. A grande aplicação deste tipo de aço é na indústria automobilística em função de sua excelente conformabilidade, aliada à característica de não envelhecimento. Estas características facilitam a produção e permitem o desenvolvimento de novos projetos. Na década de 90, a Companhia Siderúrgica Nacional (CSN) iniciou a produção em escala de materiais macios, como os aços IF através do desgaseificador à vácuo RH. Procurando aumentar a participação no mercado automotivo, foi necessário desenvolver o processo de aparamento lateral do aço IF nas Linhas de Decapagem Contínua 3 e 4 da CSN para atender a demanda dos clientes. Neste trabalho, foram identificadas variáveis que influenciam a qualidade da borda aparada lateralmente das chapas laminadas a quente e a frio, as quais estão sendo testadas e ajustadas: (1) Material do aço IF, (2) parâmetros de aparamento lateral, (3) qualidade da navalha circular de aço ferramenta e sua lubrificação,(4) velocidade da navalha circular, (5)uso ou não do push-up-roll, um equipamento auxiliar para o aparamento da borda da chapa de aço. O principal objetivo do trabalho aqui apresentado é encontrar as condições que impeçam a formação de rebarba na borda do aço IF laminado a quente e decapado após aparado lateralmente nas Linhas de Decapagem Contínua 3 e 4 da CSN. As características da apara lateral foram avaliadas por microscopia eletrônica de varredura. Análises microestruturais, bem como ensaios de tração também foram executadas em amostras coletadas nas chapas laminadas a quente e a frio para vários parâmetros definidos de aparamento lateral. / The Interstitial Free (IF) steels have been used all over the world in an increasing way since the 70 decade\'s. The great application of this type of steel is in the automobile industry in function of your excellent formability, allied to the non aging property. These characteristics facilitate the production and allow the development of new projects. In this work, it has been identified variables that influence the quality of side trimming edge of hot and cold rolled strip, which were tested and adjusted: (1) IF steel material, (2) side trimming parameters,(3) tool steel circular knife quality and your lubrication;(4) tool steel speed, (5)use or not of pushup-roll , an auxiliary equipment to the cutting edge. The main objective of the work here presented it is to find the operation conditions that impede the burr formation in the edge of hot and pickled IF steel after side trimmed in the Continuous Pickling Lines 3 and 4 at CSN. The side trimmed characteristics were assessed by scanning electron microscopy. microstructural analyses, as well as tensile tests were also performed in samples taken from the hot and cold-rolled strip for the various adopted side trimming parameters.

Os aços livres de intersticiais (IF) têm sido usados em todo o mundo de um modo crescente desde a década de 70. A grande aplicação deste tipo de aço é na indústria automobilística em função de sua excelente conformabilidade, aliada à característica de não envelhecimento. Estas características facilitam a produção e permitem o desenvolvimento de novos projetos. Na década de 90, a Companhia Siderúrgica Nacional (CSN) iniciou a produção em escala de materiais macios, como os aços IF através do desgaseificador à vácuo RH. Procurando aumentar a participação no mercado automotivo, foi necessário desenvolver o processo de aparamento lateral do aço IF nas Linhas de Decapagem Contínua 3 e 4 da CSN para atender a demanda dos clientes. Neste trabalho, foram identificadas variáveis que influenciam a qualidade da borda aparada lateralmente das chapas laminadas a quente e a frio, as quais estão sendo testadas e ajustadas: (1) Material do aço IF, (2) parâmetros de aparamento lateral, (3) qualidade da navalha circular de aço ferramenta e sua lubrificação,(4) velocidade da navalha circular, (5)uso ou não do push-up-roll, um equipamento auxiliar para o aparamento da borda da chapa de aço. O principal objetivo do trabalho aqui apresentado é encontrar as condições que impeçam a formação de rebarba na borda do aço IF laminado a quente e decapado após aparado lateralmente nas Linhas de Decapagem Contínua 3 e 4 da CSN. As características da apara lateral foram avaliadas por microscopia eletrônica de varredura. Análises microestruturais, bem como ensaios de tração também foram executadas em amostras coletadas nas chapas laminadas a quente e a frio para vários parâmetros definidos de aparamento lateral. / The Interstitial Free (IF) steels have been used all over the world in an increasing way since the 70 decade\'s. The great application of this type of steel is in the automobile industry in function of your excellent formability, allied to the non aging property. These characteristics facilitate the production and allow the development of new projects. In this work, it has been identified variables that influence the quality of side trimming edge of hot and cold rolled strip, which were tested and adjusted: (1) IF steel material, (2) side trimming parameters,(3) tool steel circular knife quality and your lubrication;(4) tool steel speed, (5)use or not of pushup-roll , an auxiliary equipment to the cutting edge. The main objective of the work here presented it is to find the operation conditions that impede the burr formation in the edge of hot and pickled IF steel after side trimmed in the Continuous Pickling Lines 3 and 4 at CSN. The side trimmed characteristics were assessed by scanning electron microscopy. microstructural analyses, as well as tensile tests were also performed in samples taken from the hot and cold-rolled strip for the various adopted side trimming parameters.

In the last decade aluminium started to be considered as an alternative to steel to produce car body panels, especially considering the strict demands to decrease fuel consumption which require vehicle weight reduction. In order to keep their leading role, steel companies have to produce stronger materials to reduce the thickness of steel sheets used in cars and are now considering non-conventional steel making processes. The purpose of this PhD research was to investigate the possibility of strengthening thin sheets of interstitial free steel (IF steel) by using carbon rich films deposited on the steel surface using Physical Vapour Deposition (PVD). These films then act as a carbon reservoir which upon heat treatment release carbon in the IF steel and strengthen it. Coated tensile coupons 200 μm thick were annealed at different temperatures under high vacuum. Tensile tests show that a 100 MPa increase in yield stress can be obtained after annealing at 430 ˚C for 1h in high vacuum. The effects of annealing environment, film thickness and prestrain on carbon diffusion were also investigated. It was shown that carbon diffusion from the film to the IF steel substrate is limited by the film transformation into cementite at temperatures equal or higher than 530 ˚C. All tensile curves showed a plastic instability known as Lüders plateau, which is undesirable as it results in surface markings on the deformed part. FEM analyses were performed to find ways to suppress the Lüders plateau, proving that increasing strain-hardening or having a graded instead of uniform carbon content through thickness can suppress or limit Lüdering. The possibility of creating a through thickness gradient of microstructure was investigated as it could suppress Lüdering and result in higher strength. For these tests, FeC coated coupons were induction heated to 820 ˚C followed by water quenching. After only 2 minutes of heat treatment the yield stress was increased by 250 MPa and the ultimate tensile strength reached 400 MPa. With an annealing of 4 minutes, the Lüders plateau was fully suppressed and the microstructure consisted in ferrite grains and TiC nanocarbides. This work demonstrates that FeC films can be effectively used to diffuse carbon into steel and that a significant increase in mechanical properties can be obtained after a heat treatment of only a few minutes.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20703号 / 工博第4400号 / 新制||工||1684(附属図書館) / 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 乾 晴行, 教授 安田 秀幸 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

With increasing fuel costs and the current drive to reduce greenhouse gas emissions and fuel consumption, a need to reduce vehicle weight is apparent. Weight reduction can be achieved by replacing conventionally stamped structural members with hydroformed parts. The weight reduction can be further enhanced by reducing the thickness of the hydroformed members through the use of advanced high strength steel (AHSS). A primary limitation in hydroforming AHSS, is the limited ductility or formability of these materials. This limitation becomes acute in multi-stage forming operations in which strain path changes become large making it difficult to predict formability. Thus, the focus of the current work is to study the effects of pre-bending on the subsequent hydroformability of Dual-Phase DP600 steel tubes. As part of this effort, the effect of key bending and hydroforming process parameters, bending boost and hydroforming end-feed, have been studied in a parametric fashion. <br /><br /> Multi-step pre-bending and hydroforming experiments were performed on 76. 2 mm (3. 0") OD tubes with a wall-thickness of 1. 85mm (DP600). Experiments were also performed on 1. 74mm Interstitial Free (IF) steel tube, which provided a low strength, high formability baseline material for comparison purposes. A fully instrumented servo-hydraulic mandrel-rotary draw tube bender was used in the pre-bending experiments in which various levels of boost were applied. The results showed that increased boost reduced the major (tensile) strain and thinning at the outside of the bend. At the inside of the bend, the compressive minor strain became larger and thickening increased. <br /><br /> Hydroforming of the straight and pre-bent tubes was conducted using various levels of load-control end-feed (EF). For both straight and pre-bend tube hydroforming, an increase in hydroforming EF resulted in increased burst pressure and corner-fill expansion (CFE). The effect of bending boost on CFE was also measured. For a given hydroforming EF case, a tube bent with greater boost achieved a higher burst pressure and consequently a greater CFE which increased the hydroformability of the material. Pre-bending was shown to consume a considerable amount of the formability of the tube in the hydroforming experiments. For the same EF case, the pre-bent tubes could only achieve a fraction of the straight tube CFE at burst. <br /><br /> The pre-bending and hydroforming experiments were complimented by finite element simulation in the hope of providing additional insight into these processes. The finite element (FE) models were able to accurately predict the strain and thickness changes imposed during pre-bending. The models were able to accurately predict the CFE, EF displacement, and strain and thickness distributions after hydroforming. <br /><br /> The extended stress-based forming limit curve (XSFLC) failure criterion was applied to predict failure (onset of necking) during hydroforming, which was measured as the burst pressure in the experiments. For straight tube hydroforming, the XSFLC predicted the correct failure pressure versus hydroforming EF load trend, but over predicted the failure pressures. In pre-bend hydroforming, the models were able to capture the effect of bending boost and hydroforming EF on the hydroformability of the tubes. The XSFLC was able to capture the drop in formability for bending versus straight tube hydroforming, but was unable to capture the failure pressure versus hydroforming EF load trend or magnitude. Further work is required to make the XSFLC applicable to straight and pre-bend hydroforming.

With increasing fuel costs and the current drive to reduce greenhouse gas emissions and fuel consumption, a need to reduce vehicle weight is apparent. Weight reduction can be achieved by replacing conventionally stamped structural members with hydroformed parts. The weight reduction can be further enhanced by reducing the thickness of the hydroformed members through the use of advanced high strength steel (AHSS). A primary limitation in hydroforming AHSS, is the limited ductility or formability of these materials. This limitation becomes acute in multi-stage forming operations in which strain path changes become large making it difficult to predict formability. Thus, the focus of the current work is to study the effects of pre-bending on the subsequent hydroformability of Dual-Phase DP600 steel tubes. As part of this effort, the effect of key bending and hydroforming process parameters, bending boost and hydroforming end-feed, have been studied in a parametric fashion. <br /><br /> Multi-step pre-bending and hydroforming experiments were performed on 76. 2 mm (3. 0") OD tubes with a wall-thickness of 1. 85mm (DP600). Experiments were also performed on 1. 74mm Interstitial Free (IF) steel tube, which provided a low strength, high formability baseline material for comparison purposes. A fully instrumented servo-hydraulic mandrel-rotary draw tube bender was used in the pre-bending experiments in which various levels of boost were applied. The results showed that increased boost reduced the major (tensile) strain and thinning at the outside of the bend. At the inside of the bend, the compressive minor strain became larger and thickening increased. <br /><br /> Hydroforming of the straight and pre-bent tubes was conducted using various levels of load-control end-feed (EF). For both straight and pre-bend tube hydroforming, an increase in hydroforming EF resulted in increased burst pressure and corner-fill expansion (CFE). The effect of bending boost on CFE was also measured. For a given hydroforming EF case, a tube bent with greater boost achieved a higher burst pressure and consequently a greater CFE which increased the hydroformability of the material. Pre-bending was shown to consume a considerable amount of the formability of the tube in the hydroforming experiments. For the same EF case, the pre-bent tubes could only achieve a fraction of the straight tube CFE at burst. <br /><br /> The pre-bending and hydroforming experiments were complimented by finite element simulation in the hope of providing additional insight into these processes. The finite element (FE) models were able to accurately predict the strain and thickness changes imposed during pre-bending. The models were able to accurately predict the CFE, EF displacement, and strain and thickness distributions after hydroforming. <br /><br /> The extended stress-based forming limit curve (XSFLC) failure criterion was applied to predict failure (onset of necking) during hydroforming, which was measured as the burst pressure in the experiments. For straight tube hydroforming, the XSFLC predicted the correct failure pressure versus hydroforming EF load trend, but over predicted the failure pressures. In pre-bend hydroforming, the models were able to capture the effect of bending boost and hydroforming EF on the hydroformability of the tubes. The XSFLC was able to capture the drop in formability for bending versus straight tube hydroforming, but was unable to capture the failure pressure versus hydroforming EF load trend or magnitude. Further work is required to make the XSFLC applicable to straight and pre-bend hydroforming.