Dissertations / Theses on the topic 'Interstitial free steel'

Doutoramento em Engenharia Mecânica
This Ph.D. research focuses on asymmetric rolling (ASR), as an alternative method for improving mechanical responses of aluminium-magnesium alloy and interstitial free (IF) steel regarding industrial requirements. Aluminium alloys are attractive materials in various industries due to their appropriate properties such as low density and corrosion resistance; however, their low formability has limited their applications. As formability of aluminium alloys can be improved through texture development, part of this dissertation is dedicated to producing the desired crystallographic texture with the ASR process. Two types of ASR (i.e. reverse and continuous asymmetric rolling) were investigated. The impact of shear deformation imposed by ASR processes on developing the desirable texture and consequently on mechanical behaviours was observed. The developed shear texture increased the normal and also planar anisotropy. Texture evolution during plastic deformation as well as induced mechanical behaviour were simulated using the “self-consistent” and Taylor models. Interstitial free (IF) steel was the second material selected in this dissertation. Since IF steel is one of the most often used materials in automotive industries it was chosen to investigate the effect of shear deformation through ASR on its properties. Two types of reverse and continuous asymmetric rolling were carried out to deform IF steel sheets. The results of optical microscopy and atomic force microscopy observations showed no significant difference between the grains’ morphology of asymmetric and conventionally rolled samples, whereas the obtained results of transmission electron microscopy indicated that fine and equiaxed dislocation cells were formed through the asymmetric rolling process. This structure is due to imposed shear deformation during the ASR process. Furthermore, the mechanical behaviour of deformed and annealed sheets was evaluated through uniaxial tensile tests. Results showed that at low thickness reductions (18%) the asymmetric rolled sample presented higher stress than that of the conventionally rolled sheet; while for higher thickness reductions (60%) the trend was reversed. The texture analyses indicated that intense rolling texture components which developed through 60% thickness reduction of conventional rolling cause a relatively higher stress; on the contrary the fine structure resulting from ASR appears to be the source of higher stress observed after pre-deformation of 18%.
A presente tese de doutoramento foi dedicada ao estudo da laminagem assimétrica (ASR) como técnica alternativa para a melhoria das propriedades mecânicas das ligas de alumínio-magnésio e aço IF durante processos industriais de conformação plástica. As ligas de alumínio são bastante atrativas devido às suas propriedades específicas, nomeadamente baixa densidade e resistência à corrosão. No entanto, a sua baixa formabilidade limita o seu campo de aplicação. A formabilidade das ligas de alumínio pode ser melhorada através da alteração da textura cristalográfica. Assim, parte desta dissertação é dedicada ao desenvolvimento de uma textura compatível com formabilidade acrescida, através da técnica ASR. A ASR foi conduzida de duas formas distintas designadas ASR – contínua e ASR – com trajetória invertida. O impacto da deformação de corte imposta pela ASR no desenvolvimento da textura e comportamento mecânico pretendidos foi analisado em detalhe. A textura cristalográfica desenvolvida produz o aumento da anisotropia planar. A evolução da textura cristalográfica foi simulada com recurso aos modelos “self-consistent” e Taylor. O aço IF foi o segundo material estudado. Dada a sua vasta utilização na indústria automóvel pretende-se investigar o efeito da deformação de corte desenvolvida durante a ASR nas suas propriedades micro e macro com o intuito de melhorar a resposta a solicitações mecânicas. No aço IF foram também estudadas as duas condições de ASR anteriormente referidas. De acordo com as observações realizadas por microscopia ótica e microscopia de força de atómica as morfologias dos grãos obtidas durante o processo ASR e laminagem convencional são semelhantes. As observações realizadas por microscopia eletrónica de transmissão revelaram que durante a ASR se formam nano células de deslocações com forma equiaxial. A estrutura desenvolvida deverá estar associada à deformação de corte imposta durante a ASR. O comportamento mecânico do aço recozido e deformado foi avaliado através de ensaios de tração uniaxial. Para valores de redução de espessura da ordem dos 18% os provetes pré-deformados por ASR apresentam valores de tensão mais elevados do que os provetes pré-deformados em laminagem convencional. Para reduções de espessura da ordem dos 60% verificaram-se resultados opostos. A análise de textura indicou que o componente de laminagem se desenvolve de forma intensa para reduções de 60%, pelo contrário, a estrutura refinada resultante da ASR parece estar na origem do valor elevado de tensão observado após pré-deformação de 18%.

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.

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.

The objectives of this study were to examine the effect of strain and path change on the microstructural evolution of ultrafine grained interstitial free (IF) steel during equal channel angular extrusion (ECAE); to determine the mechanical properties; to observe the resulting texture; and to perform optical and electron microscopy of the resulting material. The effects of different routes of extrusion (A, B, C, C' and E), heat treatment and plastic strains from 1.15 to 18.4 were examined. Monotonous tensile testing was used to determine mechanical behavior of processed materials. X-ray diffraction and TEM analyses were performed to evaluate the effect of processing on texture and grain morphology. Hardness measurements were performed to determine recrystallization behavior of the processed material. Optical microscopy was conducted on heat treated samples to determine their grain size and refinement. Monotonous tensile testing of processed materials showed that there was significant strengthening after the first extrusion. Further processing resulted in increasing values of yield strength and ultimate tensile strength, with ductility at failure varying depending upon which processing route was used. The best tensile strength results were obtained after processing Routes 8C' and 16E, due to the significant grain refinement these routes produced. X-ray diffraction revealed increases in strength of preferred texture along the directions [111] and [001], perpendicular to the transverse plane, for all specimens that were processed using ECAE. TEM observations showed a consistent refinement of grain size as the amount of processing increased, especially within Routes C' and E. Hardness measurements of heat treated specimens showed that the onset of recrystallization occurred at approximately the same temperature of recrystallization as that of pure iron, 450°C. The recrystallization curves for all samples showed that grain growth begins at a temperature of around 700°C. The low carbon content of IF steel made optical microscopy challenging. The grain size of annealed materials becomes finer and more uniform, ranging between 60 and 90 μm2, at high strain levels under Routes C' and E, due to the many potential nucleation sites developed in highly worked material.

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.

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.

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.

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.

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.

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.

Interstitial free (IF) steels are used in applications in the automotive industry where their excellent formability can be taken advantage of. This results from the removal of interstitial elements from solution by the addition of elements with a high affinity for carbon and nitrogen. Whilst highly formable, these steels have an associated low proof stresses and low dent resistance. To allow the use of thinner gauges in cars, strengthening of IF steels has been carried out by the addition of solid solution strengthening elements such as manganese, phosphorus and silicon. Despite obtaining desirable strength increases, the formability is reduced in the high strength IF steels. Formability is known to be related to crystallographic texture. High r-values (a measure of formability) are seen when advantageous {111}<uvw> orientations have a strong intensity. Orientations deviated away from the favourable types, particularly {100} types, are detrimental to formability. The development of such textures in unalloyed, formable IF steels have been extensively studied using modern techniques such as Electron Back Scatter Diffraction (EBSD). The work presented in this thesis involves the study of texture development in a typical, commercially relevant, high strength IF steel, beginning with the hot rolled material through to the fully annealed condition. The literature review outlines the metallurgy of IF steels as well as discussing the factors affecting texture in IF steels. The mechanisms operating during rolling and annealing are also reviewed. The experimental work includes evaluation of textures throughout processing from hot rolling to annealing. Emphasis is placed on the study of the evolution and development of texture during recrystallisation, using the EBSD technique. An understanding of the mechanism controlling texture evolution during recrystallisation has been developed by studying grain orientations and grain boundaries at different stages of recrystallisation. Texture evolution in the steel studied is dependent on oriented nucleation. No evidence in support of favourable orientation relationships affecting growth was found. The lower r-values seen in high strength IF steels can be explained from the textures.

Interstitial free (IF) steel is used extensively throughout applications in the automotive, packaging and furniture industries due to its excellent formability and ductility. The manufacturing process ensures excellent material properties for subsequent forming processes are developed through the formation of a fine equi-axed grain structure and crystallographic texture. The annealing process improves the formability of the cold rolled IF sheet, whilst also reducing strength through the recovery and recrystallisation process. After the cold rolling process the grain structure is heavily deformed. During the recovery process the dislocation density is reduced through annihilation and redistribution of dislocations to form sub grains. During the recrystallisation process new grains nucleate and grow into new, strain free, grains. Magnetic properties of ferromagnetic material are known to be affected by microstructural phenomena such as dislocation density, grain boundaries, grain size and texture. It is therefore possible to monitor the recovery and recrystallisation processes using sensors that are responsive to changes in magnetic properties. The purpose of the research completed was to establish whether it would be possible to use electromagnetic (EM) sensors to monitor recovery and recrystallisation processes in-situ during heat treatment, such that EM sensors could then be deployed in a continuous annealing line.

Interstitial Free (IF) Steels are widely used in the automotive industry. Their excellent mechanical property characteristics, such as the high formability typified by high r_mean-values, low yield strength and high elongation make them very suitable for manufacturing into automotive body panels. However, their properties and microstructures are affected by their composition and processing conditions. This research program is focused on the optimisation of the properties of two Interstitial Free Steel Grades, an experimentally produced mild Ti-Nb IF Grade and a commercially produced high strength Ti-IF grade. The optimisation of the processing conditions is vital to the enhancement of obtained properties and is investigated in two parts within this research program. The first part will simulate the effect of variable hot rolling parameters on these two grades. This study will aim to investigate the effect of the finishing temperature, effect of run-out table cooling and coiling temperature, all of which are extremely important parameters within the hot mill and may have a significant effect on the microstructure and properties of the two grades. For this work the Gleeble 3500 thermomechanical system has been employed to simulate the hot rolling conditions. The second part will aim to simulate the effect of batch annealing sequences on cold rolled strips of these two grades. This shall aim to determine the optimum annealing conditions to obtain desirable properties on the studied grades. The work demonstrated the differences in microstructure and properties of the two grades investigated, with respect to the simulated processing parameters. It could be observed that the microstructures obtained on these two grades were indeed very different, with the microstructure of the high strength commercial Ti-IF grade consisting of mainly non-equilibrium ferritic and / or bainitic structures, while the microstructure of the mild experimental Ti-Nb IF Grade mainly consisting of recrystallised equiaxed ferrite. Testing of the batch annealed strips, revealed that the r_mean -values were very high for both steels, thus permitting a determination of the annealing temperature for which optimum strength and formability values could be obtained.

A demanda da indústria automobilística por aços de boa estampabilidade, para aplicação em peças expostas motivou o desenvolvimento de várias especificações de aços livres de intersticiais (IF) na Companhia Siderúrgica Nacional. A especificação de aço \"IFTi\", pertencente ao grupo de aços para estampagem extra profunda especial (CSN EEPIF), com limite de resistência < 380 MPa está, dentre todos os graus de aço processados nas linhas de laminação a frio, entre os que apresentam a maior porcentagem de desvio por marca de rebarba e outros problemas relacionados ao corte. A geração de uma rebarba com altura excessiva está relacionada a um corte de má qualidade. Durante o processo de laminação a frio essa rebarba pode aumentar ainda mais de tamanho, se desprender, marcar os cilindros de trabalho e conseqüentemente aumentar os custos de produção por desvios e troca precoce de cilindros. Nas linhas de decapagem, a tesoura de pontas final é o equipamento responsável por separar as bobinas soldadas no início do processo e definir a característica de corte das pontas e caudas das mesmas, através de corte por guilhotinamento. Visando aumentar a qualidade, produtividade, diminuir as perdas durante o processo e atender às exigências dos clientes internos e externos, foi necessário desenvolver o processo de corte por guilhotina do aço IF nas tesouras de pontas final das Linhas de Decapagem Contínua 3 e 4 da CSN. Através da simulação de corte em laboratório, usando-se a mesma velocidade de avanço e ângulo de inclinação do equipamento de campo, avaliou-se a relação entre o ajuste de folga entre navalhas e o comportamento força x deslocamento no cisalhamento. Além disso, através da análise por microscopia eletrônica de varredura (MEV) foi possível observar o efeito dos varios ajustes de folga na superfície de fratura, identificando as regiões de corte citadas na literatura e sua relação com as características microestruturais e com as propriedades do material. Constatou-se que a tensão cisalhante durante o corte diminuía a medida em que a folga aumentava de forma linear para uma faixa de folga ajustada entre 6 e 12%t em amostras de aço IF-Ti com 1,5mm de espessura. Nas micrografias feitas em MEV, observou-se que as interfaces entre penetração e fratura ficaram irregulares em todos os casos, sendo uma região intermediária, com aspecto misto entre as regiões de penetração e fratura foi relacionado ao desencontro das frentes de trincas principalmente devido ao grande ângulo de fratura. Tanto os perfis de corte quanto as superfícies de fratura mostraram que a fração da região de corte diminuía com o aumento da folga, até certo ponto, quando a partir da mudança do comportamento da tensão cisalhante, voltava a aumentar. Pode-se verificar também, que a fração da região de deformação aumentava à medida que folgas maiores eram testadas e os valores atingidos eram muito superiores as deformações esperadas para as respectivas características do tipo de corte indicado pelas regiões de penetração e fratura. Os melhores resultados de corte foram obtidos na regulagem de folga entre 4 e 8%t, com rebarbas inferiores a 1,5%t, menores frações da região de deformação e interface corte/fratura mais regulares do que nas demais faixas de folga.
The automotive industry\'s demand for high formability steels for use in exposed body panels motivated the development of several interstitial free (IF) steels specifications at Companhia Siderurgica Nacional. The \"IF-Ti\" steel specification belongs to the special deep drawing steels group (CSN EEP-IF), with yield strength <380 MPa, and among all grades of steel processed in the cold rolling lines is the one with the highest amount of deviation by burr marks and other problems related to shearing. The generation of an excessive burr height is related to a poor cut quality. During the cold rolling process the burr can increase in size, break off, mark the work rolls and therefore increase production costs per deviation and early exchange of cylinders. In the pickling lines, the exit shear is the device responsible for separating the coils welded at the beginning of the process and to define the cutting head and tail characteristics, either through guillotine cutting. Aiming to increase quality, productivity, reduce losses during the process and meet the requirements of internal and external customers, it was necessary to develop the guillotine cutting process of the IF steel sheet extremities (head and tail) in the exit shears of Continuous Pickling Lines #3 and #4 at CSN. Through the cutting simulation in the laboratory, using the same forward speed and rake angle adopted in the field equipment, was evaluated the relationship between the adjustment of clearance between blades and the behavior shear force versus displacement. In addition, through the analysis by scanning electron microscopy (SEM) it was possible to observe the effect of several settings on the fracture surface, identifying the different regions cited in the literature and its relation to the microstructural characteristics and to the material properties. It was found that the shear stress during cutting reduced as the gap increased linearly to a range between 6 and 12%t in samples of IF-Ti steel with 1.5 mm thick. In the micrographs taken by SEM, it was observed that the interfaces between burnished and fracture were irregular in all cases, where an intermediate depth, with mixed aspect between the burnished and fracture depths was related to mismatch of crack fronts mainly due to the large fracture angle. Both cut profiles as the fracture surfaces showed that the burnished depth decreased with increasing the gap to some extent, since from the behavior change of shear stress, it started to increase. It was also possible to verify that the fraction of therollover depth increased as larger gaps were tested and the values achieved were much higher than expected for the rollover characteristics of the respective type of cut indicated by the burnished and fracture depths. The best cutting results were obtained for clearance adjustment between 4 and 8%, resulting in burrs below 1.5%t, smaller rollover depth and more regular burnished/fracture interface than in other clearance ranges tested.

碩士
國立中山大學
材料與光電科學學系研究所
98
The IF steel was cycled at strain amplitude of εmax = 0.3% and the samples were cycled at 3000、6000 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 10μm. 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 2μm. 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.

碩士
國立高雄應用科技大學
模具工程系
106
In present, the construct of vehicles or home appliances use IF steel as sheet metal, however, the production costs are very high, even with variable advanced technique to control the size of spangles. Hence, in this paper, I use different abrasive brush roller in processing of HDG in the same condition to produce steel sheets with different roughness, then ignore other possible parameters because of the similar producing condition. In this research, I focus on two main questions: Discussing the interconnection between the scale of spangles, sheet resistance, as well as wet fastness by setting the roughness as the main parameter, and studying the effect of intermetallic compound when HDG steel enters iron-aluminum phase with zinc bath in molten condition. Based on this research, I find out that roughness really gets the interconnection to the scale of spangles, sheet resistance, as well as wet fastness: With the steel rougher, the scale of spangles would be smaller, and sheet resistance as well as wet fastness show in direct proportion. In addition, I check the second question by SEM (Scanning Electron Microscope) and XRD (X-ray diffraction). The former points out that the scale of iron-aluminum crystallite has no correlation to the roughness of substrate; the latter can only indicate that this molten condition pushes HDG steel to enter Iron-aluminum phase rather than Iron-zinc phase. After further investigating the crystallographic plane produced in Iron-aluminum phase, this produce has no correlation to the roughness of substrate.

碩士
國立臺灣大學
材料科學與工程學研究所
106
Ultra-low carbon IF steels (0.001-0.002 wt% C) were studied in the present research. Hollow cylinder specimens were used to obtain rapid cooling by the high resolution dilatometer. The transformation mechanism was analyzed by dilatometric measurement. The transformation temperature of massive ferrite and the critical cooling rate in corresponding steel were obtained. Optical microscopy (OM), electron backscattered diffraction (EBSD), and transmission electron microscope (TEM) were used to realize the effects of cooling rate and characterize grain morphologies of massive ferrite. The fraction of massive ferrite, containing lots amount of substructure, increases with the cooling rate. Dislocation density of massive ferrite was measured and is higher than that in polygonal ferrite. In the second part, the research focuses on the inelastic mean free path measurement by using electron energy-loss spectrometry (EELS). Log-ratio method is used widely in the thickness measurement. The inelastic mean free path was measured under both TEM and STEM mode. Different experimental conditions were operated. The mean free path decreases with the increasing of semi-collection angle in both TEM and STEM mode. The convergent angle effect is more significant under STEM mode, and the results show that lager convergent angle make the mean free path small. The effects of dislocation and amorphous layer have also been revealed.

Interstitial-Free (I-F) steels are increasingly being used for press forming operations due to their markedly improved deep-drawability. Additional interest is due to the fact that the cold rolled I-F steels can be effectively heat treated in a continuous annealing line without the need for any accompanying overaging process. The objective of this study was to characterize the evolution of microstructure and crystallographic texture of a 80 % cold rolled, Ti-stabilized I-F steel during heating rates applicable to batch and continuous annealing processes. Isothermal recovery kinetics, as monitored by {220} x-ray peak resolution measurements, were described using a semi-empirical logarithmic equation. Isothermal recrystallization kinetics were determined by quantitative metallographic measurements and were characterized by both Johnson-Mehl-Avrami-Kolmogorov and Speich-Fisher relationships. The isothermal recrystallization kinetics were also described in terms of the experimentally determined microstructural path function and an empirical kinetic function relating the interfaceaveraged growth rate with recrystallization time. The additivity procedure was success fully employed in conjunction with the isothermal kinetic parameters to predict continuous heating recovery and recrystallization kinetics at heating rates simulating batch and continuous annealing processing. Microstructural examination showed that the recrystallization event was heterogeneous and related to the cold rolled cell structure. The recrystallized nuclei, developed primarily by subgrain coalescence occurring during the later stages of recovery, grew into the cold rolled matrix by the migration of high misorientation boundaries. Large precipitates of TiN and TiS acting as preferred nucleation sites and fine precipitates impeding the boundary mobility were also observed. The hot band texture with the moderate presence of (112)[1ï0] yielded a strongly developed cold rolled texture extending from (001)[1ï0] to (112)[1ï0]. The recrystallization texture was characterized by the strong presence of (554)[225] and (111) [1ï0]. Grain growth following recrystallization strengthened the existing texture with an accompanying improvement in average strain ratio values. The effect of heating rate on the final recrystallization texture was found to be insignificant.

碩士
國立中山大學
材料與光電科學學系研究所
99
This study is mainly aimed at interstital-free and dual-phase steels, analyzing the compositions and distribution of selective surface oxides after annealing and then to know the influence of these oxidation for the formation of FeAl inhibition layer in hot-dip galvanizing. Interstital-free and dual-phase steels were first annealed at 800 oC for 1-200 s in a 10% H2-N2 protected atmosphere of -70 oC and 0 oC dew point respectively and then dipped in zinc bath with Al content 0.12-0.18 wt% for 0-20 s. Using this combined SEM, Auger electron spectroscopy(AES), X-ray photoelectron spectroscopy(XPS) and ICP-AES etc. instruments, it is shown that the MnAl2O4 spinels were the major oxidation on the surface of IF steel after annealing. The average oxidation thickness was about 5-15 nm. Annealing times has little effect on the thickness. On the other hand, MnO were observed on DP steel surface after anneaing. The MnO paticles mainly distributed at the grain boundaries ,and the average oxdaiton thickness increase rapidly from 20 nm(10 s) to 110 nm(200 s) with annealing times. The growth of the FeAl inhibition layer can separate to nucleation in initial stage and diffusion growth later. The Fe2Al5 nucleation times were all about 0.1 s in both steels , and average thicknesses were approximately 20 nm. For IF steels , Al uptake in the zinc bath and steel interface was depleted in nucleation stage with 0.12 wt% Al content, so that delayed the growth of Fe2Al5, and the rate determining step was the diffusion of Al in zinc bath. When Al content raise up to 0.14 wt%, the phenomenon of growth delay was not happened, and the rate determining step of Fe2Al5 growth changed to the solid-state diffusion of Fe in Fe2Al5. For DP steels, when Al content up to 0.14 wt%, the growth mechanism was similar to IF steels, but the rate determining step of Fe2Al5 growth was mainly in the grain boundary diffusion of Fe in Fe2Al5. Moreover, where the MnO paticles was rich could obviously observe the delay of Fe2Al5 growth. It was probably because of consuming a great deal of Al to reduce the MnO oxides.