Limiting dome height (LDH) tests were used to evaluate the formability of both base metal and laser butt welded blanks of AHSS (including High strength low alloy (HSLA), Dual phase (DP) steels of different grades). Mechanical properties of the base metal and welded blanks were assessed by uniaxial tensile and biaxial LDH tests, and related to measured microhardness distributions across the welds. The formability ratio of laser welded dual phase sheet steels generally decreases with increased base metal strength. A significant decrease of LDH was observed in the higher strength DP steel welded specimens due to the formation of a softened zone in the Heat Affected Zone(HAZ). Softened zone characteristics were correlated to the LDH. Larger softened zones led to a larger reduction in the LDH. HAZ softening has been shown to be a function of the base metal martensite content and the weld heat input. Formability also decreased with increased weld heat input. Both in experiment and numerical simulations strain is localized in the softened HAZ in the uniaxial tensile testing, indicating that strain localization decreases tensile strength and elongation of laser welds in DP980.

Limiting dome height (LDH) tests were used to evaluate the formability of both base metal and laser butt welded blanks of AHSS (including High strength low alloy (HSLA), Dual phase (DP) steels of different grades). Mechanical properties of the base metal and welded blanks were assessed by uniaxial tensile and biaxial LDH tests, and related to measured microhardness distributions across the welds. The formability ratio of laser welded dual phase sheet steels generally decreases with increased base metal strength. A significant decrease of LDH was observed in the higher strength DP steel welded specimens due to the formation of a softened zone in the Heat Affected Zone(HAZ). Softened zone characteristics were correlated to the LDH. Larger softened zones led to a larger reduction in the LDH. HAZ softening has been shown to be a function of the base metal martensite content and the weld heat input. Formability also decreased with increased weld heat input. Both in experiment and numerical simulations strain is localized in the softened HAZ in the uniaxial tensile testing, indicating that strain localization decreases tensile strength and elongation of laser welds in DP980.

Tailor welded blanks (TWBs) are used in the automotive industries as a method to meet economic, environmental and governmental demands. Conventionally, TWBs incorporated mild and low strength steels such as interstitial free and draw quality steels because of their excellent formability traits. However, due to their low strength they are unsuitable for energy absorption applications; thus, the interest of incorporating advanced high strength steels (AHSS) into the TWBs. Dual phase (DP) steel is a type of AHSS that is of interest because of its combination of high strength and good formability that is comparable to high strength low alloy (HSLA) steels. However, welding DP steel causes softening in the heat affected zone (HAZ), which leads to premature failure and reduces formability. The aim of this thesis was to study the effect of weld design on the formability of TWBs with DP steels and with HSLA steel. This thesis is divided into three parts; the first part examines TWBs with different weld line positions, weld line orientations and strain paths. The second part investigates bead-on plate curvilinear blanks and its effect on formability of the blanks. The last part examines the effects of multiple welds on the formability of TWBs.

Tailor welded blanks (TWBs) are used in the automotive industries as a method to meet economic, environmental and governmental demands. Conventionally, TWBs incorporated mild and low strength steels such as interstitial free and draw quality steels because of their excellent formability traits. However, due to their low strength they are unsuitable for energy absorption applications; thus, the interest of incorporating advanced high strength steels (AHSS) into the TWBs. Dual phase (DP) steel is a type of AHSS that is of interest because of its combination of high strength and good formability that is comparable to high strength low alloy (HSLA) steels. However, welding DP steel causes softening in the heat affected zone (HAZ), which leads to premature failure and reduces formability. The aim of this thesis was to study the effect of weld design on the formability of TWBs with DP steels and with HSLA steel. This thesis is divided into three parts; the first part examines TWBs with different weld line positions, weld line orientations and strain paths. The second part investigates bead-on plate curvilinear blanks and its effect on formability of the blanks. The last part examines the effects of multiple welds on the formability of TWBs.

In the present study, the influence of the quenching temperature and partitioning conditions (temperature and time) have been investigated on a 0.6%C-1.2%Mn-1.6%Si-1.75%Cr alloy. Maps of hardness, impact toughness and amount of retained austenite have been developed for three quenching temperatures as a function of partitioning temperature and partitioning time. Results demonstrate that, in this material, the carbon depletion of the martensite and the stabilization of austenite can be achieved significantly faster at high partitioning temperatures, promoting higher retained austenite fractions, lower hardness, and maximizing the energy absorbed in a Charpy V‑Notch test. In addition, the effect of the partitioning time was also analysed, presenting different behaviour at high and low partitioning temperatures. Whereas an increment of time at high partitioning temperatures (>400 ºC) leads to an austenite consumption, at low partitioning temperatures it is effective to retain a higher amount of austenite.  Furthermore, tensile properties are shown to be better than in conventional alloys utilized in industry. Whilst, for example, the AISI 52100 alloy achieves 2 GPa of tensile strength and 1‑2% of fracture deformation, in the present study the notable combination of 2.5 GPa of tensile strength and 5.7 % of fracture deformation was achieved in samples quenched until room temperature. Untempered martensite transformed during final cooling in samples quenched until higher temperatures was shown to be detrimental for tensile properties. A comparison between the Q&P process and the austempering process on this alloy has been carried out. Results reveal that the quenching and partitioning heat treatment is presented as a promising alternative to reach higher hardness (>700 HV) and similar specific wear rates in dry conditions performing a shorter heat treatment.    Finally, a complementary study about the effect of micro-segregation on the Q&P process and an optimization method to minimize the inhomogeneity of the structure by a correct selection of the quenching temperature were established.

No description available.

No description available.

Dual-phase (DP) advanced high-strength steels (AHSSs) are widely used in the automotive industry due to their excellent combination of strength, ductility, and work hardening properties. However, defects occurring during processing make these ferrous alloys expensive. Toward this ends, high-temperature tensile tests using a Gleeble thermomechanical simulator have been conducted to determine the stress/strain behaviour at temperatures between 1250 to 1480 C in order to quantify the tensile strength and ductility. The results of both as-cast and transfer-bar material will be presented as well as three different sample geometries in order to better understand the effects of starting microstructure, thermal gradient, and tress/strain distribution on the reproducibility of high temperature properties. Optical and scanning electron microscopy are then performed to further elucidate the structure/property relationships. The results show that the presence of preexisted prorosities in the as-cast structure decreases the high-temperature strength of the material, while the transfer-bar samples show lower ductility at ultra-high temperatures, (T 1450 C), due to their severe susceptibility to melting. In terms of the two mentioned thermomechanical characteristics, voids nucleation, growth, and coalescence initiated with porosity clustering are the main mechanisms behind the lower strength of the as-cast samples, whilst tearing apart of the melt plays an important role to drastically drop the ductility of transfer-bars at mentioned temperature interval. Moreover, the long-gauge-length (LGL) geometry proposes better reproducibility of data compared with the other geometries. This is attributed to a suitable combination between low stress localization and high thermal gradient during the Gleeble testing that provides a condition in which the samples experience sharp localized necking right on the hot-spot zone. The obtained data can be used as part of multi-physics process and microstructure continuous casting models. / Thesis / Master of Applied Science (MASc)

Medium Mn (med-Mn) third generation advanced high strength steels (3G AHSSs) are promising candidates for meeting automotive weight reduction requirements without compromising passenger safety. However, the thermal processing of these steels should be compatible with continuous galvanizing line (CGL) processing capabilities as it provides cost-effective, robust corrosion protection for autobody parts. Hence, the main objective of this Ph.D. research is to develop a CGL-compatible thermal processing route for a prototype 0.2C-6Mn-1.5Si-0.5Al-0.5Cr-xSn (wt%) (x = 0 and 0.05 wt%) med-Mn steel that will result in the 3G AHSS target mechanical properties (24,000 MPa%  UTS × TE  40,000 MPa%) and high-quality galvanized coatings via enhanced reactive wetting. It was found that the starting microstructure, intercritical annealing (IA) time/temperature, and Sn micro-alloying had a significant effect on the retained austenite volume fraction and stability and, thereby, the mechanical properties of the prototype med-Mn steel. For the as-received cold-rolled (CR) starting microstructure, the intercritical austenite nucleated and grew on dissolving carbide particles and resulted in blocky retained austenite. However, Sn micro-alloying significantly effected the intercritical austenite chemical stability by segregating to the carbide/matrix interface and retarding C partitioning to the intercritical austenite. This resulted in lower volume fractions of low stability retained austenite which transformed to martensite (via the TRIP effect) at low strains, thereby quickly exhausting the TRIP effect and resulting in a failure to sustain high work hardening rates and delay the onset of necking. Consequently, the Sn micro-alloyed CR starting microstructure was unsuccessful in achieving 3G AHSS target mechanical properties regardless of the IA parameters employed. Contrastingly, the CR starting microstructure without Sn micro-alloying was able to meet target 3G mechanical properties via intercritical annealing at 675 °C × 60 s and 120 s, and at 690 °C × 60 s owing to sufficiently rapid carbide dissolution and C/Mn partitioning into the intercritical austenite such that it had sufficient mechanical and chemical stability to sustain a gradual deformation-induced transformation to martensite and maintain high work hardening rates. On the other hand, the martensitic (M) starting microstructure produced higher volume fractions of chemically and mechanically stable lamellar retained austenite regardless of Sn micro-alloying. Intercritical annealing at 650 °C × 60 s and 675 °C × 60 s and 120 s produced 3G AHSS target mechanical properties. It was shown that the stable lamellar retained austenite transformed gradually during deformation. Furthermore, deformation-induced nano-twin formation in the retained austenite was observed, suggesting the TWIP effect being operational alongside the TRIP effect. As a result, a continuous supply of obstacles to dislocation motion was maintained during deformation, which aided in sustaining a high work hardening rate and resulted in a high strength/ductility balance, meeting 3G AHSS target properties. Based on these results, the martensitic starting microstructure without Sn micro-alloying and the M-675 °C × 120 s IA condition were chosen for the selective oxidation and reactive wetting studies. The selective oxidation study determined the effect of a N2-5H2-xH2O (vol%) process atmosphere pO2 (–30, –10, and +5 °C dew point (Tdp)) on the composition, morphology, and spatial distribution of the external and internal oxides formed during the austenitizing and subsequent intercritical annealing cycles. The objective of this study was to identify the process atmosphere for the promising M-675 °C × 120 s heat treatment that would result in a pre-immersion surface that could be successfully galvanized in a conventional galvanizing (GI) bath. The austenitizing heat treatment (775 °C × 600 s) used to produce the martensitic starting microstructure resulted in thick (~ 200 nm) external oxides comprising MnO, MnAl2O4, MnSiO3/Mn2SiO4, and MnCr2O4, regardless of the process atmosphere pO2. However, intermediate flash pickling was successful in dissolving the external oxides to a thickness of approximately 30 nm along with exposing metallic Fe in areas which contained relatively thin external oxides. Furthermore, extruded Fe nodules that were trapped under the external oxides were revealed during the flash pickling process. Overall, flash pickling resulted in a surface consisting of dispersed external oxide particles with exposed metallic substrate and extruded Fe nodules. This external surface remained unchanged during IA owing to the multi-micron (~ 2–8 µm) solute-depleted layer that formed during the austenitizing heat treatment. Subsequent galvanizing in a 0.2 wt% (dissolved) Al GI bath with an immersion time of 4 s at 460 °C was successful in achieving high-quality, adherent galvanized coatings through multiple reactive wetting mechanisms. The dispersed nodule-type external oxides along with exposed substrate and extruded Fe nodules on the pre-immersion surface facilitated direct wetting of the steel substrate and promoted the formation of a robust and continuous Fe2Al5Znx interfacial layer at the steel/coating interface. Additionally, oxide lift-off, oxide wetting, bath metal ingress, and aluminothermic reduction were operational during galvanizing. The galvanized med-Mn steels met 3G AHSS target mechanical properties. Overall, this Ph.D. research showed that it is possible to employ a CGL-compatible thermal processing route for med-Mn steels to successfully produce 3G AHSS target mechanical properties as well as robust galvanized coatings. / Thesis / Doctor of Philosophy (PhD) / One of the largest challenges associated with incorporating the next generation of advanced high strength steels into the automotive industry lies in processing these steels in existing industrial production lines. In that regard, a two-stage heat treatment with an intermediate flash pickling stage and process atmosphere compatible with existing industrial continuous galvanizing line technology was developed for a prototype medium-Mn steel. The heat-treated prototype steel met the target mechanical properties outlined for the next generation of advanced high strength steels. Furthermore, the heat treatment and process atmosphere utilised in this research produced a surface that facilitated the successful galvanizing of the prototype medium-Mn steel. This adherent and high-quality galvanized coating will provide robust corrosion protection if the candidate medium-Mn steel is used in future automotive structural applications.

The diploma thesis deals with the determination of the structure of AHSS steels and offers solutions in the form of new observation methods with the aid of a scanning electron microscope using filtration of slow secondary electrons. The thesis describes electron filtration in order to display secondary electrons in a certain energy range which carry a surface information that is not normally visible. The advantages and benefits of such observation are clearly demonstrated.