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In response to more stringent global CO2 emissions, automotive manufacturers have increased the use of advanced high strength steels (AHSS). Ultra-high strength steels are often used within the body-in-white (BIW) for safety critical parts and structural reinforcements, such as roof rails and side impact beams. Currently, the most commonly used press hardened steel (PHS) grade for these applications is 22MnB5, with a typical composition of 0.22C 1.2Mn 0.25Si 0.005B (wt%). Automotive OEMs have expressed a desire to use Zn-based coatings as they are compatible with the current painting system and have the potential to provide robust cathodic protection. The steel blanks generally undergo direct hot press forming (DHPF) to achieve the necessary martensitic microstructure and target mechanical properties, but this presents challenges for Zn-coated 22MnB5. The adoption of Zn-based coatings within the automotive industry has been inhibited by the prospect of liquid metal embrittlement (LME) resulting from DHPF, as well as the desire to provide robust cathodic protection. Previous literature has reported that a zinc ferrite (α-Fe(Zn)) coating with a global Zn content of at least 30 wt% will provide cathodic protection to the underlying substrate. The main goal of this work was to determine the microstructural evolution and electrochemical properties of galvanized (GI70 – 70 g/m2/side) 22MnB5 substrates as a function of the annealing time at a typical austenization temperature of 900°C. It was found that the Zn-based coatings annealed at 700°C consisted to a mixture of small volume fraction of α-Fe(Zn) and Г-Fe3Zn10. After heating to 900°C, the coating comprised varying volume fractions of α-Fe(Zn) and Zn(Fe) liquid, which transformed to Г-Fe3Zn10 after solidification. The relative fraction of Г Fe3Zn10 was found to decrease with increasing annealing time until the coating completely transformed to α-Fe(Zn) after annealing at 900°C for 240 s. GDOES results found that, when the sample was annealed at 900°C for 240 s, the global Zn content of the coating was less than 30 wt%. Coatings comprising varying fractions of Г-Fe3Zn10 were subjected to uniaxial tensile tests to determine how the coating microstructure affected the mechanical properties in comparison to the uncoated substrate material. It was found that the uncoated substrate material met the mechanical property requirements of σ(UTS)min ≥ 1500 MPa regardless of annealing time. However, σ(UTS) was found to decrease with increasing annealing times for the GI70 coated samples until the target mechanical properties were not met when the sample was annealed at 900°C for 180 s. This was attributed to increased coating thicknesses leading to a decrease in the martensitic cross-sectional area to support the load. Furthermore, the coatings were subjected to a variety of electrochemical characterization techniques, including potentiodynamic and galvanostatic polarization scans, potentiostatic scans, and electrochemical noise tests. Potentiodynamic polarization scans indicated a higher driving force for cathodic protection when the coating contained some fraction of Г-Fe3Zn10. Furthermore, a limiting current density for these samples was observed, demonstrating that Г-Fe3Zn10 corrodes at a slower rate in comparison to α Fe(Zn). Galvanostatic polarization measurements indicated that, when the fraction of Г Fe3Zn10 within the coating was below 15 vol%, the protective properties of the phase were not exhibited. XRD and TEM analysis revealed the formation of three corrosion products on the surface: simonkolleite, hydrozincite, and akaganeite. It was found that, when samples contained greater than 15 vol% Г-Fe3Zn10 in the coating, the predominant corrosion products were a combination of simonkolleite and hydrozincite. When the Г Fe3Zn10 content was below this value, the dominant corrosion product was found to be akaganeite. Furthermore, substrate attack was observed on a sample annealed at 900°C for 420 s when the coating layer was intact, indicating that the α-Fe(Zn) only containing coating obtained at this time does not provide cathodic protection. Based upon the current results, it was determined that a minimum volume fraction of 15 vol% Г-Fe3Zn10 must be present within the coating layer to obtain robust cathodic protection. Furthermore, it was determined that the processing window to develop cathodically protective Zn based coatings while mitigating LME is extremely narrow. This is a result of the fact that it is necessary for at least 15 vol% Г-Fe3Zn10 to be present within the coating microstructure at room temperature, which is liquid at the forming temperatures of 900°C. From the current findings, it was found that it is unlikely that a cathodically protective Zn-based coating can be obtained for DHPF steel parts using 22MnB5 as a substrate material. This is due to the high forming temperature resulting in liquefication of the coating and the rapid cooling rates necessary to achieve the target mechanical properties of σ(UTS)min ≥ 1500 MPa. Thus, it is recommended that the current substrate material be altered such that the part may be formed below the peritectic temperature of 782°C. / Thesis / Master of Applied Science (MASc)

Yes / Tensile properties of powder metallurgy 3% manganese-0·8% carbon (content of green compact) steels were determined following laboratory sintering in (nearly) full, semiclosed containers with no getter powders in dry, 0-100% hydrogen-nitrogen atmospheres. Manganese was mixed with the NC 100·24 sponge iron powder as low carbon ferromanganese and carbon as a graphite addition. Dogbone compacts were pressed at 660 MPa, the sintering temperatures were 1120 and 1250°C and cooling rates ∼65 K min- 1. In specimens sintered in nitrogen containing atmospheres at 1120°C, final carbon content was ∼0·7% and for those processed at 1250°C ∼0·6%. Sintering in dry hydrogen resulted in lower carbon and oxygen contents. Independent of the H2/N2 ratio in the furnace atmosphere, however, all the specimens were ductile and exhibited similar strengths. Yield strengths R 0·2 were in the range: 426-464 MPa, tensile strengths Rm were 724-780 MPa and strains to failure were 1·6-2·0% after sintering at 1250°C. The 1120°C sintering temperature resulted in 10-15% lower strength values. The microstructures, significantly devoid of oxide networks, comprised mainly mixtures of bainite and fine (divorced) pearlite, with very little martensite and retained austenite. Reproducibly successful sintering of manganese containing compacts requires that reduction conditions exist at the sintering temperature. Ellingham Richardson diagrams dictate that the dewpoints of hydrogen required are-55 and-40°C at 1120 and 1250°C, respectively. A semiclosed container, how ever, ensures a different microclimate. It is suggested that then the initial relevant reactions there are: Mn[vapour]+H2O=MnO+H2, 3Fe2O3 +H2= 2Fe3O4+H2O, Fe3O4+H2=3FeO+H2O, FeO+H2= Fe+H2O and C+O2=CO2, which provide hydrogen andwater vapour,also within the pores. The manganese vapour further acts as a ‘shield’ by generating further hydrogen from the water vapour. The following reactions involving carbon monoxide are postulated above 927°C, when CO is a more effective reducing agent than hydrogen: C+H2O=H2+CO, 3Fe2O3+ CO=2Fe3O4+CO2, Fe3O4+CO=3FeO+CO2, FeO+CO=Fe+CO2 and C+CO2=2CO. Accordingly, irrespective of whether it is hydrogen or nitrogen in the semiclosed container, if there is a supply of carbon, reducing conditions prevail at the sintering temperature,embrittling oxidenetworks arenot formed and ductile manganese steels are processed.

The rise of Zn-coatings on direct press hardened steels for body-in-white passenger safety applications over the widely used Al-Si coatings is due to its lower cost, compatibility with Zn-based paint systems, and offers sacrificial cathodic protection in addition to barrier protection. Manufacturing the complexly-shaped high strength automotive parts using the direct hot press forming method (DHPF) transforms the Zn-based coating into a mixture of Γ-Fe3Zn10 and α-Fe(Zn). Previous literature has determined that a minimum of 15 vol% Γ-Fe3Zn10 is required within the coating to provide robust cathodic protection of the steel substrate. This assumed the mixed potential theory is valid for modeling the electrochemical properties of the mixed phase coating; however, the interwoven coating phase morphology results in varying volume fractions of Γ-Fe3Zn10 and α-Fe(Zn). Potentiodynamic polarization scans of GI70 coated 22MnB5 steel annealed at 890°C for various annealing times revealed that Γ-Fe3Zn10 + α-Fe(Zn) coatings with at least 11 vol% Γ-Fe3Zn10 exhibit electrochemical properties insignificantly different from those comprising pure Γ-Fe3Zn10, and behaves similarly to pure α-Fe(Zn) for coatings with less than 11 vol% indicating that the Γ-Fe3Zn10 + α-Fe(Zn) coatings behave as a homogeneous single phase, thus validating the use of the mixed potential theory. Scanning vibrating electrode technique analysis of various galvanic couples determined that Γ-Fe3Zn10 provides strong cathodic protection for the 22MnB5 steel and moderate protection for α-Fe(Zn), while the 22MnB5 steel is only weakly protected by α-Fe(Zn). Separation of the 22MnB5 steel and Γ-Fe3Zn10 by an intermediary α-Fe(Zn) layer reduces the cathodic protection of the 22MnB5 steel since the α-Fe(Zn) layer acts as an electron receptor and limits the macroscale throwing power of Γ-Fe3Zn10. / Thesis / Master of Applied Science (MASc) / Zn-coatings on direct press hardened steels are designed to electrochemically protect the steel substrate from corrosion. Manufacturing automotive parts using the direct hot press forming method transforms the Zn-based coating into a two-phase mixture of Γ-Fe3Zn10 and α-Fe(Zn). The previously determined minimum 15 vol% Γ-Fe3Zn10 required for the coating to provide the steel substrate robust protection assumed that mixed potential theory was a valid model to predict the electrochemical properties of the two phase coating, despite its complex microstructure. It was found that the use of mixed potential theory was valid as it was determined that the Γ-Fe3Zn10 + α-Fe(Zn) coatings behave as a single phase, with robust cathodic protection of direct hot pressed coatings requiring a minimum of 11 vol% Γ-Fe3Zn10. It was further determined that Γ-Fe3Zn10 cathodically protects both the steel and α-Fe(Zn), while α-Fe(Zn) only weakly cathodically protects the steel.

The formation of nano–sized precipitates is considered to be the origin of hardening and embrittlement of ferritic steel used as structural material for pressure vessels of nuclear reactors, since these nanoclusters hinder the motion of dislocations within the grains of the polycrystalline bcc–Fe matrix. Previous investigations showed that these small precipitates are coherent and may consist of Cu, Ni, other foreign atoms, and vacancies. In this work a combination of on–lattice simulated annealing based on Metropolis Monte Carlo simulations and off–lattice relaxation by Molecular Dynamics is applied in order to determine the structure, energetics and thermodynamics of coherent clusters in bcc–Fe. The most recent interatomic potentials for Fe–Cu–Ni alloys are used. The atomic structure and the formation energy of the most stable configurations as well as their total and monomer binding energy are calculated. Atomistic simulation results show that pure (vacancy and copper) as well as mixed (vacancy-copper, copper-nickel and vacancy-copper-nickel) clusters show facets which correspond to the main crystallographic planes. Besides facets, mixed clusters exhibit a core-shell structure. In the case of v_lCu_m, a core of vacancy cluster coated with copper atoms is found. In binary Cum_Ni_n, Ni atoms cover the outer surface of copper cluster. Ternary v_lCu_mNi_n clusters show a core–shell structure with vacancies in the core coated by a shell of Cu atoms, followed by a shell of Ni atoms. It has been shown qualitatively that these core–shell structures are formed in order to minimize the interface energy between the cluster and the bcc-Fe matrix. Pure nickel consist of an agglomeration of Ni atoms at second nearest neighbor distance, whereas vacancy-nickel are formed by a vacancy cluster surrounded by a nickel agglomeration. Both types of clusters are called quasi-cluster because of their non-compact structure. The atomic configurations of quasiclusters can be understood by the peculiarities of the binding between Ni atoms and vacancies. In all clusters investigated Ni atoms may be nearest neighbors of Cu atoms but never nearest neighbors of vacancies or other Ni atoms. The structure of the clusters found in the present work is consistent with experimental observations and with results of pairwise calculations. In agreement with experimental observations and with recent results of atomic kinetic Monte Carlo simulation it is shown that the presence of Ni atoms promotes the nucleation of clusters containing vacancies and Cu. For pure vacancy and pure copper clusters an atomistic nucleation model is established, and for typical irradiation conditions the nucleation free energy and the critical size for cluster formation have been estimated. For further application in rate theory and object kinetic Monte Carlo simulations compact and physically–based fit formulae are derived from the atomistic data for the total and the monomer binding energy. The fit is based on the structure of the clusters (core-shell and quasi-cluster) and on the classical capillary model.

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.

Made available in DSpace on 2014-06-11T19:28:35Z (GMT). No. of bitstreams: 0 Previous issue date: 2005-06Bitstream added on 2014-06-13T18:34:54Z : No. of bitstreams: 1 ranieri_a_me_guara.pdf: 1386692 bytes, checksum: c47cd01ee98e83332bd255dbd49cadad (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Universidade Estadual Paulista (UNESP) / Os principais objetivos deste trabalho foram desenvolver estruturas bainíticas e multifásicas através de diversas rotas de tratamentos térmicos, visando as melhores combinações de propriedades mecânicas, fornecendo subsídios científicos/tecnológicos para as indústrias brasileiras. Em certos componentes de veículos aeroespaciais tem sido usado aço de baixa liga e ultra-alta resistência temperados e revenidos com elevada resistência devida a estrutura martensítica mas com baixa tenacidade. Uma melhoria na tenacidade é conseguida com redução controlada de resistência através do revenimento. O novo conceito, para aços avançados que combinam alta resistência com boa tenacidade, está simbolizado pelas microestruturas bainíticas e multifásicas. Neste projeto foi feito um estudo do efeito das microestruturas nas propriedades mecânicas de um aço AISI 4340. Foram analisadas diversas microestruturas, desde aquelas inteiramente bainíticas até microestruturas multifásicas com teores variados de ferrita, bainita, martensita e austenita retida. Os resultados foram comparados com aqueles obtidos por têmpera por resfriamento continuo e com as diversas rotas de transformação isotérmica. As combinações de propriedades mecânicas estão relacionadas com as frações volumétricas das fases e a bainita melhora significativamente a ductilidade do aço, mantendo a resistência elevada e melhorando a combinação resistência/ductilidade. O aço possui baixo coeficiente de encruamento e é possível conseguir resistências entre 1000 MPa e 1400 MPa com alongamento entre 13% e 25%, combinação esta superior aquelas encontradas para o mesmo aço quando temperado e revenido em óleo. / The main goals of this study were to develop bainitic and multiphasic structures through several routes of heat treatment, in order to reach the better combination of mechanical properties, providing scientific/technological subsidies to Brazilian industries. In some of aerospatial vehicles components have been used quenched and tempered ultra-high-strength low-alloy steel where the martensitic structure is responsible for the high-strength and low toughness levels. Toughness improvements can be achieved by strength reduction control during tempering. The new concept for advanced steels, that combine high-strength and good toughness, is correlated with the bainitic and multiphasic microstructures. In this work the effect of microstructures on the mechanical properties of AISI 4340 steel. Has been analysed several microstructures, from those totally bainitic until multiphasics microstructures with various ferrite, bainite, martensite and retained austenite content. The results were compared with those obtained by quenching through continuous cooling transformation and several routes of isothermal transformation. The combinations of mechanical properties are related with volume fraction of present phases and the bainite improved significantly the toughness steel., keeping the high strength and improving the strength/toughness combination. This steel has low coefficient of hardness and is possible to achieve strengths between 1000 MPa e 1400 MPa with percentual elongation between 13% e 25%, this combination is better than that found to the same steel when quenched and tempered in oil.

Made available in DSpace on 2014-10-09T12:28:54Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:57:01Z (GMT). No. of bitstreams: 0 / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP