Carlos González-Rivera

Processing of ultra low carbon steels with mechanical properties adequate for automotive applications in the as-annealed condition

A series of ultra low carbon/Ti added steels were produced with the aim of evaluating the steelmaking route and processing conditions of slabs, in order to achieve mechanical properties on resulting annealed sheets adequate for automotive applications. Characterization of microstructure was carried out in the as-cast, deformed and annealed specimens by means of scanning and transmission electron microscope techniques. Slab specimens were hot rolled, coiled, cold rolled and annealed. From the resulting annealed sheets, evaluation of mechanical properties in terms of the percent of elongation and the normal anisotropy ratio was carried out on three experimental ultra low carbon/Ti added steels from heats of 230 t. From the resulting mechanical properties, it was noticed that the sheet steels achieved very high formability, especially for parts requiring good deep drawability, adequate for automotive applications.

Solidification kinetics of a near eutectic Al-Si alloy, unmodified and modified with Sr

The purpose of this work was to explore the differences in solidification kinetics between unmodified and Sr modified eutectic Al-Si alloy as revealed by Fourier Thermal Analysis (FTA) and grain-growth kinetics characterization. Thermal analysis were performed in cylindrical stainless steel cups coated with a thin layer of boron nitride, using two type-K thermocouples connected to a data acquisition system. Grain growth kinetics characterization was carried out using solid fraction evolution and grain density data. FTA results for the non modified and modified alloys suggest that there are changes in the solidification rate during eutectic nucleation followed, during growth, by similar solidification rate evolutions, suggesting that this parameter is governed principally by the heat extraction conditions. On the other hand the change of the grain growth parameters estimated for the experimental probes suggest that the presence of Sr may modify the relationship between grain growth rate and undercooling in eutectic Al-Si.

Effect of the Presence of SiCp on Dendritic Coherency of Al–Si-Based Alloys During Solidification

In order to explore the effect of the presence of SiC particles on dendritic coherency during solidification of Al–Si based metallic alloys, a factorial fractional two levels experimental design was implemented to allow an identification of the main effects of the particle content, silicon content, grain refinement, and cooling rate on the solid fraction at coherency. This solidification parameter was determined for Al-3wt%Si and Al-7%Si alloys, and Al-3%Si/SiCp and Al-7%Si/SiCp metal matrix composites. The cooling process during solidification was monitored by performing cooling curve measurements at two radial locations within samples poured into cylindrical molds at two cooling rates. The experimental cooling curves were numerically processed by the Fourier thermal analysis method to know the evolution of solid fraction as a function of time. The effect of grain size was included using samples with or without grain size refinement. The grain refinement was obtained by adding predetermined quantities of TiAlB master alloy. It was found that presence of SiC particles affects the coherency point of the metal matrix composites increasing the solid fraction at coherency. However this effect is relatively small when compared to the effect of grain refinement, cooling rate, and Si content on dendritic coherency of experimental probes.

On the local microstructural characteristics observed in sand cast Al–Si alloys

Abstract The aim of this work is to explore the phenomenology that describes the local solid formation and heat transfer occurring during sand cast alloys solidification in order to propose an explanation to the observed changes of local microstructural characteristic lengths in hypoeutectic and eutectic Al–Si based alloys. Microstructural observations are made in different radial positions of solidified rod castings. Also, solidification kinetics information is obtained using the Fourier thermal analysis method. A coupled heat transfer-solidification kinetics model is employed to predict the thermal history, the solidification kinetics and some microstructural parameters in order to compare the predictions with experimental results. The model and the experimental outcome suggest that there is a strong dependence of the local solidification kinetics on the local heat transfer. The analysis of this dependence is used to propose an explanation to the observed changes in microstructural characteristics at different locations within sand castings.

Slab cracking after continuous casting of API 5L X-70 grade steel for pipeline sour gas application

AbstractAn API 5L X-70 grade steel for large diameter pipeline application with sour gas resistance was developed. The resulting Fe–C–Mn–Nb slabs were controlled rolled and accelerated cooled. In most of the slabs, mechanical properties equivalent to the above grade were broadly achieved. However, despite achieving excellent mechanical properties, during the test period, several slabs exhibited cracks in the cooling yard, causing their rejection. To elucidate the nature of the crack, several specimens were analysed by employing optical, scanning electron, and transmission electron microscopy. Microstructural characterisation together with microanalysis and thermal analysis carried out on as cast specimens showed the presence of rodlike and/or dendritelike precipitates of the Fe2 Nb type associated with the cracks. The elimination of the cracks was achieved by increasing the niobium content in the Nb ferroalloy to ensure its dissolution in the liquid bath after its addition during the alloying stage.

Novel Degasification Design for Aluminum Using an Impeller Degasification Water Physical Model

A full scale water physical model of an aluminum degassing crucible was built to analyze the effect of changing the point of gas injection, from the conventional location, through the shaft, to an injection underneath impeller, on the performance of three rotor designs, based on the oxygen kinetic elimination from water by N2 injection, similar to hydrogen removal from liquid aluminum, under different degassing conditions. Results show that the point of gas injection plays an important role in the kinetics and efficiency of the degassing operation. The use of gas injection underneath the impeller increases notoriously the efficiency of the degassing processes.

Effect of Process Variables on Kinetics and Gas Consumption in Rotor-Degassing Assisted by Physical and Mathematical Modeling

Mathematical and physical models of water deoxidation in a batch aluminum degassing reactor using the rotor-injector technique were developed. The mathematical model was successfully validated against measured degassing kinetics. The physical model was employed to perform a process analysis using a two-level factorial experimental design to determine the influence of gas flow rate, impeller angular velocity, and gas injection points on gas consumption efficiency and degassing kinetics. A combination of higher rotor speeds and gas flow rates results in fast degassing kinetics. However, moderate gas flow rates are recommended to save gas.

Newton Thermal Analysis of Gray and Nodular Eutectic Cast Iron

The purpose of this work is to analyze the capability of Newton Thermal Analysis (NTA) to detect differences of the solidification kinetics between eutectic gray and ductile irons obtained from the same base alloy. The NTA experimental output has been analyzed with a simple micromacro modeling approach. The outcome of this work suggests that NTA has a good potential as a qualitative tool to characterize the solidification kinetics of alloys.

Optimizing gas stirred ladles by physical modeling and PIV measurements

ABSTRACT The hydrodynamic performance of a gas-stirred ladle is analyzed experimentally, through a factorial experimental design, through physical modeling and particle image velocimetry techniques, to find the effects of the nozzle position, and its number, the gas flow rate and the presence or not of an oil layer on the flow velocity field, the turbulent kinetic energy distribution, and the open eye area, A. Flow patterns changed drastically in the presence of one or two plugs and as a result of the nozzle’s positions. The turbulent flow fields showed significant differences under the presence or absence of an oil layer and the open eye area strongly depends on the gas flow rate, the position and the number of nozzles. Responses , , and A were statistically processed to identify the relevant effects of each variable on , , and A. These responses were used to calculate the optimum process conditions that minimize A and maximize both and , and then improve mixing efficiency in ladles using multi-objective optimization through the genetic algorithm (GAmultiobj). A Pareto characterization and its analysis were performed to reveal the embedded operation rules allowing optimum performances of the ladle for specific production goals.

Mathematical modeling of a gas jet impinging on a two phase bath

In this work a three phase 3D mathematical model was developed using the Volume Of Fluid (VOF) algorithm, which is able to accurately describe the cavity geometry and size as well as the liquid flow patterns created when a gas jet impinges on a two phase liquid free surface. These phenomena are commonly found in steelmaking operations such as in the Electric Arc Furnace (EAF) and the Basic Oxygen Furnace (BOF) where oxygen jets impinge on a steel bath and they control heat, momentum and mass transfer. The cavity formed in the liquids by the impinging jet depends on a force balance at the free surface where the inertial force of the jet governs these phenomena. The inertial force of the jet and its angle play important roles, being the lowest angle the best choice to shear the bath and promote stronger circulation and better mixing in the liquids.