Radioactive gold has been added to the liquid pool during the continuous casting of mild steel billets, blooms and beam blanks and of a stainless steel slab. The tests were conducted on low-head, curved mold and straight mold, vertical bend type casting machines. From autoradio-graphs of the sections of the steel, observations were made of the flow pattern in the liquid pool, of the uniformity and thickness of the solid shell in the mold and sub-mold regions and of the cast structure of the strand. Pool depths were estimated from the position of tungsten pellets containing radioactive cobalt, dropped into the pool with the gold. One and two-dimensional finite difference heat transfer models were developed to calculate the pool profiles in strands being continuously cast. The predicted pool profiles and pool depths have been compared to profiles measured from autoradiographs and pool depths measured with tungsten pellets. The model-predicted surface temperatures of low carbon steel billets at the mold bottom have been compared to measured values reported in the literature. The pool and surface temperature profiles calculated with the finite difference model have been compared to profiles predicted by an integral profile model. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Growing economical and ecological requirements have driven the automotive industry towards lowering fuel consumption by reducing vehicle weight and in the meantime maintaining safety standards. This pushes the development of advanced light weight alloys. As a promising candidate for automotive applications, the twinning induced plasticity (TWIP) steels have received tremendous attention due to their superior mechanical properties, which combines high ultimate tensile strength and ductility simultaneously. However, their plasticity mechanism and recovery behavior are still not completely understood. In the first part of this investigation, a modified physical-based model is developed based on the recent observations of strong influence of grain orientation and Bauschinger effect. The model describes well the serrated plastic flow of Fe-22Mn-0.6C TWIP steel under quasi-static tensile loading at ambient temperature. The discrete twinning is considered responsible for the serration phenomenon in Fe-22Mn-0.6C TWIP steel. The model also provides a new approach to estimate the volume fraction of deformation twins. In the second part of this investigation, the recovery of pre-strained Fe-22Mn-0.6C TWIP steel is studied. A model describes the evolution of yield stress during annealing is proposed, which indicates that the recovery treatment does not remove deformation twins induced by cold rolling but decreases dislocation density in matrix and removes the dislocations piled up against twin and grain boundaries. The recovery activation energy rec Q is derived from experimental results. The value Qrec = 1.73 eV = 167 kJ/mol implies that the diffusion from dislocations cores governs the recovery of investigated TWIP steel. / published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

Thesis (MTech (Mechanical Engineering))--Peninsula Technikon, 2002 / Phase transformations in steel are of profound commercial importance due to the fact that they influence the physical and mechanical properties of the end product. During the casting process, which occurs at temperatures in excess of 1500°C, the microstructure of the steel is austenitic. During slow cooling the austenite will decompose into lower temperature microstructures such as ferrite, pearlite or hard, brittle cementite, the latter becoming increasingly present the higher the carbon content of the steel. Steel manufacturers, however, make use of additional forming processes such as rolling, quenching and reheat or tempering to impart specific mechanical or metallurgical properties to a particular steel. If the cooling rate is increased, as would be the case during a quenching process, the austenite will decompose forming a needle like microstructure called bainite in addition to the other microstructures. A further increase in the cooling rate will result in the diffusionless transformation of part of the austenite to form martensite, a hard brittle microstructure. If heat was now to be applied to the steel, as in a tempering process, some of the martensite formed during the quenching process will again transform into a softer microstructure such as pearlite or ferrite.

In this research, the effects of gas composition, elapsed time of reaction and temperature on scale formation and descaling of low carbon steel were investigated and results were discussed from the viewpoint of the phase composition of the scales, oxidation rates, oxidation mechanisms, adhesion, fracture mechanics, porosity and residual scale. / The phase composition and morphology of scales grown under conditions similar to those of reheating furnaces were analyzed. Low carbon steel was oxidized over the temperature range 1000-1250°C in gas mixtures of O 2-CO2-H2O-N2, O2-H 2O-N2 and O2-CO2-N2. The mole fraction of each phase, wustite (FeO), magnetite (Fe3O 4) and hematite (Fe2O3) was determined by the direct comparison method Two types of scales were observed. The first type was a crystalline scale with an irregular outer surface composed mostly of wustite, and a negligible amount of magnetite. The second type was the classical three-layer scale composed of wustite, magnetite and hematite. In general, the experiments showed that the furnace atmosphere, oxidation time and temperature influence the phase composition of the scales. / Low carbon steel was oxidized in air over the temperature range 600-1200°C for 120 s to approximate the formation of secondary and tertiary scale in hot rolling. The mole fraction of wustite, magnetite and hematite was determined by the direct comparison method The phase composition of the scales changed with temperature and time. During the initial 30 s of oxidation, wustite was the predominant phase in the temperature range 800-1200°C, and as oxidation proceeded, the percentages of magnetite and hematite increased. In addition, the texture of the scales was investigated by orientation imaging microscopy (OIM); it was found that temperature influences the texture of the scales. The experiments indicated that 850°C is the ideal temperature for the finishing mill in order to reduce surface defects and work roll wear. / The adhesion of scales formed in air on low carbon steel in continuous heating and isothermal conditions was investigated with a four-point bending test. The separation (crack) always occurred inside the scale indicating that the strength of the scale is lower than the strength of the scale/steel interface. It was found that scale adhesion is related to scale porosity, blister formation and stresses acting in the scale. A spallation process was observed when cooling from 800°C to room temperature. The microscopic observations revealed that spallation followed route 1 "strong interface and weak oxide." / Low carbon steel was oxidized over the temperature range 1050-1250°C in O2-CO2H2O-N2 gas mixtures in order to study the hydraulic descaling process. The oxidation times were 15-120 min. and the scales were 130-2000 mum thick. The experimental parameters were chosen to approximate scale formation under conditions similar to those of reheating furnaces. In the hydraulic descaling tests, two modes of scale removal were observed. In the first mode, observed in classical three-layer scales that developed an inner porous layer with low or medium porosity, the horizontal undercutting occurred at the boundary of the inner porous layer and dense scale. The second mode was observed in classical three-layer scales that developed an inner porous layer with high porosity and in crystalline scales. In the second mode, the horizontal undercutting occurred at the first plane of large pores relative to the scale/steel interface. In general, the experiments showed that scale morphology controlled the removability of scale.