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The project elaborated in frame of Master's studies branch 2303T002 presents a study of welded joints for hot-dip galvanizing. On the basis of literature study it is assumed that zinc influences the properties of welded joints. Samples were made of high strength steels DOMEX and wear-resistant steels HARDOX which were welded using MAG. Subsequently, half of the samples treated with zinc. The evaluation is used tear test and test Vickers hardness. Finally, the project is the comparison of the measured values of galvanized steel and steel without zinc.

This master’s thesis deals with the properties of highstrength steels, mainly with their resistance to corrosion. It describes discusses different types of corrosion and the possibility of protecting materials against corrosion. The thesis also evaluates compares the cost of corrosion protection by using coating and hot dip galvanizing. It describes describes the principles of degreasing and pickling, which is used as an adjustment before hot dip galvanizing. It evaluates there are many procedures for hot dip galvanizing, such as wet, dry or spin method, which is used for small parts. This thesis also evaluates the change of mechanical properties of steels: Hardox 500, Weldox 700 and 500 Weldox after hot dip galvanizing.

The main focus of the thesis is the proposal for the construction of welding jig for the extension of the bus. Welding of individual parts of the assembly is on manual workplace. All parts are made of fabricated structural steel ČSN 41 1373 with galvannealed coating. As an appropriate technology for welding of extension with the optimal parameters method MAG was chosen.

This master´s thesis deals with hot dip galvanizing of steel constructions and their protection from corrosion. This work describes the origin of corrosion of steel and its corrosion in different areas and presents degreasing and pickling before hot dip galvanization. It describes possibilities of making zinc layer by the wet and draw method of hot dip galvanization. Furthermore, it puts forward reactions between zinc and iron, possible defects of the zinc layer and influence of the hot dip galvanizing on the steel constructions. The experimental part examines the influence of the hydrogen charging on the material and the time of the flux operation on the quality of layer. The work further analyses the changing of mechanical properties of steels Hardox 500 and Weldox 700 as a result of the hydrogen embrittlement during pickling by the hydrochloric acid.

Third-generation advanced high-strength steels (3G AHSS) are being developed to assist in vehicle light weighting so that fuel efficiency may be improved without sacrificing passenger safety. 3G-AHSS have received significant interest from the automotive industry as a critical candidate for their unique combination of high strength and ductility. However, due to selective oxidation of the principal alloying elements such as Mn, Si, Al, and Cr at the steel surface during the annealing stage prior to immersion in the galvanizing Zn(Al, Fe) bath, the process of continuous hot-dip galvanizing of these steel is challenging. This thesis determined the influence of annealing process parameters such as oxygen partial pressure and annealing time, on the selective oxidation and reactive wetting of an Fe-0.15C-5.56Mn-1.17Si-1Al (wt%) prototype 3G AHSS during intercritical annealing as well as continuous galvanizing. Simulated annealing and galvanizing were conducted on the prototype Fe-0.15C-5.56Mn-0117Si-1Al (wt%) 3G steel; Intercritical annealing heat treatments were carried out at 690˚C in a N2-5 vol pct H2 process atmosphere under dew points of 223 K (–50 °C), 243 (–30 °C) and 268 K (–5 °C). MnO was the major oxide formed at the outmost layer of the external oxides on all annealed samples. The experimental parameters, on the other hand, had a substantial impact on the morphology, distribution, thickness, and surface oxide coverage. The greatest Mn surface concentration as well as maximum surface oxide coverage and thickness was obtained by annealing the panels under the 223 K (–50 °C) and 243 (–30 °C) dp process atmospheres. The oxides formed under these process atmospheres largely comprised coarse, compact, and continuous film nodules. In contrast, MnO nodules formed under the 268 K (–5 °C) dewpoint process, exhibited wider spacing between finer and thinner nodules, which was consistent with the internal oxidation mode, while under 223 K (–50 °C) dp process atmosphere, generally external oxidation took place. Poor reactive wetting was obtained for the panels annealed under the 223 K (–50 °C) dp process atmosphere for both the 60 s and 120 s holding times as well as the 243 K (–30 °C) dp process atmosphere for 120 s. This was attributed to the formation of a thick, compact oxide layer on the steel surface, which acted as a barrier between the substrate and Zn bath, preventing Fe dissolution from the substrate surface for the formation of the desired Fe2Al5Znx interfacial layer. However, a well-developed interfacial Fe-Al intermetallic layer was formed under the 268 K (–5 °C) and 243 (–30 °C) dp process atmospheres for intercritical annealing times of 60 s, which is indicative of a good reactive wetting since the thinner and nodule-like oxides on the steel surface after annealing encourage the reactive wetting. External oxides morphology plays a dominant role in facilitating the contact between Zn-alloy bath and the substrate via different mechanisms such as aluminothermic reduction which occurred for the sample annealed under the 268 K (–5 °C) dp process atmosphere. / Thesis / Master of Applied Science (MASc)

<p> Intercritically annealed steels, such as dual-phase steels, have found widespread use in automotive structural components due to their high strength and ductility. Elements such as Mn, Al and Si, added to improve the mechanical properties are selectively oxidized during heat treatment and limit the ability of the alloy to be reactively wet during continuous hot-dip galvanizing. Subsequently, a limit has been placed on the amount of alloy which can be used if the steel is to be subsequently galvanized. The specifics of this limit have not been explored in detail, nor has the mechanism of decreased wettability been well demonstrated in the literature other than to say that the galvanizing reaction is limited by oxides on the surface.</p> <p> Using a force balance, it is shown that the presence of MnO on the surface of steels greatly reduces the wettability with a typical galvanizing bath (Zn-0.2wt%Al, Fe-saturated, 460°C). Furthermore, it was determined that this is caused by the additional and rate-limiting step of aluminothermic reduction of the oxide layer with the bath Al, required for subsequent inhibition layer formation. By using a low pO2 during annealing, the wettability was improved by reducing the thickness of the MnO layer when compared to intermediate and industrially common values of pO2. Using a high pO2 also resulted in improved wettability since the internal oxide which was formed did not reduce the wettability since it was not exposed to the bath alloy.</p> <p> Improvements in hardenability were also explored via dilatometry showing that the formation of bainite is delayed with increasing Mn content, as well as a decrease in transformation temperatures from γ during cooling (i.e. Ms and Bs). At ~5wt% Mn, only the the transformation to αM could be observed. This opens the door to higher strength, galvanized steels - as well as possibly galvanized martensitic steels.</p> / Thesis / Master of Applied Science (MASc)

<p> Aluminum is usually added to the zinc bath to form an Fe-Al interfacial layer which retards the formation of a series of Fe-Zn intermetallic compounds during the hot-dip galvanizing process. However, experimentally exploring the inhibition layer formation and obtaining useful experimental data to understand the mechanisms is quite challenging due to short times involved in this process. In this study, a galvanizing simulator was used to perform dipping times as short as O.ls and rapid spot cooling techniques have been applied to stop the reaction between the molten zinc coating and steel substrate as quickly as possible. In addition, the actual reaction time has been precisely calculated through the logged sample time and temperature during the hot-dipping process. The kinetics and formation mechanism of the inhibition layer was characterized using SEM, ICP and EBSD based on the total reaction time. For bath containing 0.2wt% dissolved AI, the results show that FeA13 nucleates and grows during the initial stage of the inhibition layer formation and then Fe2Als forms by a diffusive transformation. The evolution of the interfacial layer formed in a zinc bath with 0.13wt% dissolved AI, including Fe-Aland Fe-Zn intermetallic compounds, was a result of competing reactions. In the initial period, the Fe-Al reaction dominated due to high thermodynamic driving forces. After the zinc concentration reached a critical composition in the substrate grain boundaries, formation of Fe-Zn intermetallic compounds was kinetically favoured. Fe-Zn intermetallic compounds formed due to zinc diffusing to the substrate via short circuit paths and continuously grew by consuming Fe-Al interfacial layer after samples exited the zinc bath due to the limited Al supply. A mathematical model to describe the formation kinetics as a function of temperature for the 0.2wt% Al zinc bath was proposed. It indicated that the development of microstructure of the interfacial layer had significant influence on the effective diffusion coefficient and growth of this layer. However, the model underestimates the AI uptake by the interfacial layer, particularly at higher temperatures. This is thought to be due to the effect of the larger number of triple junctions in the inhibition layer leading to an underestimation of the effective diffusivity. </p> / Thesis / Master of Science (MSc)

Third generation advanced high-strength steels (3G-AHSS) have received significant interest from leading auto steel industries and OEMs as candidate materials for reduced mass Body In White (BIW) components due to their unique combination of high specific strength and ductility. However, the continuous hot-dip galvanizing of these steels is challenging due to selective oxidation of the main alloying elements such as Mn, Si, Al and Cr at the steel surface during the annealing step prior to immersion in the galvanizing Zn(Al, Fe) bath, as extensive coverage of the substrate surface by these oxides is detrimental to reactive wetting, good coating adhesion and integrity. Simulated galvanizing treatments were conducted on two prototype Fe-0.1C-6Mn-2Si (wt pct) 3G steels; one as the reference steel and the other with 0.05 wt pct Sn added to the composition. The combined effects of annealing temperature, time, process atmosphere oxygen partial pressure and 0.05 wt pct Sn addition on the selective oxidation of the steel substrates were determined. Subsequently, the reactive wetting of the steels with respect to the pre-immersion surface structures of the samples annealed for 120 s was examined. Annealing heat treatments were carried out at 800˚C and 690˚C in a N2-5 vol pct H2 process atmosphere under three dew points of –50˚C, –30˚C and +5˚C, covering process atmosphere oxygen partial pressures within the range of 1.20  10-27 atm to 1.29  10-20 atm. MnO was present at the outmost layer of the external oxides on all samples after annealing. However, the morphology, distribution, thickness and surface coverage were significantly affected by the experimental variables. Annealing the reference steel under the low dew point process atmospheres, i.e. –50˚C and –30˚C, resulted in the highest Mn surface concentration as well as maximum surface oxide coverage and thickness. The oxides formed under these process atmospheres generally comprised coarse, compact and continuous film forming nodules, whereas the surface morphologies and distributions obtained under the +5˚C dew point process atmosphere, which was consistent with the internal oxidation mode, exhibited wider spacing between finer and thinner MnO nodules. The grain boundary internal oxide networks had a multi layer structure with SiO2 and MnSiO3 at the oxide cores and shells, respectively. Significant morphological changes were obtained as a result of Sn addition. The continuous film-like external MnO nodules were modified to a fine and discrete globular morphology, with less surface coverage by the oxides and reduced external oxide thickness. Both the external and internal oxidations followed parabolic growth kinetics, where the depth of the internal oxidation zone decreased with Sn addition and decreasing oxygen partial pressure. Poor reactive wetting was observed for the reference steel substrates that were annealed for 120 s under the –50˚C and –30˚C dew point process atmospheres at 800˚C and under the –50˚C dew point atmosphere at 690˚C, such that no integral metallic coating was formed after the 4 s immersion in the Zn(Al, Fe) bath. By contrast, excellent coating quality was obtained for the Sn-added steels when the –30˚C and +5˚C dew point process atmospheres were employed when annealing at 690˚C. The remainder of the experimental conditions demonstrated good reactive wetting with intermediate coating quality. For the two reference steels annealed at 800˚C under the –50˚C and –30˚C dew point process atmospheres, poor reactive wetting was due to full coverage of the surface by 116 nm and 121 nm thick and continuous MnO films. In the case of the 690˚C  –50˚C reference steel with the external layer thickness of only 35 nm, however, poor wetting was attributed to substantial coverage of the surface by continuous, film-like oxides. In both cases, exposure of the underlying substrate to the bath alloy and an intimate contact between the substrate Fe and the bath dissolved Al could not take place and the formation of the Fe2Al5Znx interfacial layer was hidered. For the processing conditions that satisfactory reactive wetting was obtained despite the pre-immersion selective oxidation of the surfaces, several reactive wetting mechanisms were determined. For the samples with a sufficiently thin external MnO layer, good reactive wetting was attributed to partial reduction of MnO by the bath dissolved Al, as well as bridging of the Mn sub-oxides by the Zn coating or Fe2Al5Znx interfacial intermetallics. Partial or full formation of the Fe2Al5Znx interfacial layer was observed in the successfully galvanized substrates with Fe-Al crystals formed between, underneath and also on top of the reduced oxides. Furthermore, for cases with widely-spaced, fine oxide nodules, it was found that the liquid bath alloy was able to infiltrate the external oxide/substrate interface, resulting in surface oxide lift-off and enhanced coating adhesion. It was globally concluded that the thin, discrete and fine globular morphology of external MnO, resultant of annealing the steel substrates with 0.05 wt pct Sn addition under the process atmosphere oxygen partial pressures consistent with internal oxidation, allowed for an enhanced reactive wetting by the Zn(Al, Fe) galvanizing bath which was manifested by increased amount of Al uptake and population of the Fe2Al5Znx intermetallics at the coating/steel interface. / Thesis / Doctor of Science (PhD)

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.