B. Pereda

Study of Recrystallization in High Manganese Steels by Means of the EBSD Technique

The softening processes that take place after hot deformation in two high Mn steels, one of them microalloyed with 0.1%Nb, have been investigated. Double hit torsion tests were carried out in order to determine the mechanical softening at temperatures ranging from 900 to 1100°C. In addition, the applicability of different parameters obtained by means of the Electron Back Scattering Diffraction (EBSD) technique to estimate the recrystallized fraction was investigated. It has been found that the Grain Orientation Spread (GOS) is the parameter that best allows quantifying the recrystallized fraction under the conditions investigated. The correlation between the mechanical softening and the recrystallized fraction measured by EBSD depends on the material and deformation conditions. A good correlation between both values is observed for the base steel at all the temperatures. However, for the Nb microalloyed steel, although at high temperatures a good correlation is also observed, at low temperatures the mechanical softening fraction tends to be larger than the recrystallized fraction denoting that recovery has an important contribution.

Validation and Analysis of the Parameters for Reconstructing the Austenite Phase from Martensite Electron Backscatter Diffraction Data

This work focuses on the validation of a method for reconstructing the fcc crystallographic data from martensite orientation electron backscatter diffraction (EBSD) maps based on the “γ nuclei identification” and “γ nuclei spreading strategy.” To that end, an Fe-30Ni alloy was employed. The martensite transformation start temperature (Ms) of this material is close to or below room temperature; therefore, during hot deformation and after water quenching, it presents an fcc austenitic microstructure, while after subzero quenching, austenite-to-martensite transformation takes place. Accordingly, the reconstruction procedure can be applied to the martensitic EBSD crystallographic data, and the morphological and orientation results of the reconstruction can be validated by comparison with the original crystallographic fcc data. Torsion tests were performed to produce recrystallized and deformed austenite microstructures. Although applying the Kurdjumov–Sachs orientation relationship (OR) resulted in reconstructed area fractions larger than 75 pct, the reconstruction quality improved significantly when other ORs closer to the Greninger–Troiano OR were applied. The analysis carried out on the recrystallized microstructure shows that the method is robust against variation in the different parameters involved in the reconstruction. Good angular and morphological reconstruction results were obtained in both recrystallized and deformed microstructures, including the ability to reconstruct twins.

Modelling the Static Recrystallization Kinetics of Microalloyed TWIP Steels with Different Alloying Contents

During hot rolling, austenite recrystallization determines the grain size evolution and the extent of strain accumulation, and therefore, it can be used to tailor the microstructure and mechanical properties of the final product. However, at the moment, models describing the recrystallization kinetics of high-Mn steels are scarce and they do not take into account the effect of the alloying elements present in these steels. The aim of this work is to provide a quantitative model for the determination of the static recrystallization kinetics valid for a wide range of high-Mn steel compositions. Softening data determined for steels with different Mn (20 to 30%), Al (0 to 1.5%) and C (0.2 to 1%) levels at different strain, strain-rate and temperature conditions were analyzed. Static recrystallization of the investigated high-Mn steels follow Avrami’s law, with n Avrami exponents which are temperature dependent and lower than those determined for low C steels. A dependence of the t0.5 (time for 50% fractional softening) on the carbon content has been also observed and it was incorporated into an equation for the calculation of this parameter.

Relevance of Static and Dynamic Recrystallizations on Austenite Grain Refinement in Nb-Mo Microalloyed Steels

Composite systems composed of nanocrystalline apatites and oligolactide-based polymer networks were prepared resulting in malleable and even injectable formulations which can be cured to compact materials at body temperature. Porous devices with inter-connective porosity were obtained after addition of suitable foaming agents to the composite mixtures. Setting time, porosity and mechanical properties of the composites can be properly adjusted by varying the educt composition. The determined compressive strengths and Young’s moduli of the porous composites perfectly match the mechanical characteristics of cancellous bone material. Preliminary in vitro cell culture experiments with compact composite materials demonstrated their good cytocompatibility. Based on these findings, the synthesized nano-structured composites represent promising candidates for the development of new biomaterials usable in hard tissue regeneration.

Analysis of the Static Recrystallization Behavior of Nb-Ti Microalloyed Steels Including Low Strain Levels

Semi-empirical models for predicting the austenite static recrystallization behavior are widely used in designing thermomechanical treatments to improve final mechanical properties. However, a problem with these models is that their utility can be limited to the range of deformation conditions and chemical compositions they were developed for. This work focuses on the study of the applicability of current recrystallization models to the range of low strain conditions and/or high Nb microalloying additions (≈0.1%). To do so, the recrystallization behavior of two low carbon Nb-Ti microalloyed steels (0.04 and 0.11% Nb and ≈0.01% Ti) has been investigated by torsion tests. Experimental results for recrystallization time and recrystallized grain size have been compared to previously developed equations. It has been observed that at low strains (ε = 0.1) the predictions fail. A dependence of the n Avrami exponent both on temperature and applied strain was also found.

Role of Mo on Static Recrystallization Kinetics in Coarse Grained Nb Microalloyed Steels

Thin slab direct rolling (TSDR) technology has some important metallurgical differences compared to conventional routes. One of these differences deals with the combination of coarse as-cast austenite grains and the possibility of limited solubility of microalloyed addition at the entry of the rolling mill. The industrial tendency of increasing the strength and toughness of steel grades produced by TSDR route requires increased amounts of Nb addition to achieve a proper austenite conditioning prior to transformation. Nevertheless, an increase in the Nb content rises the risk of premature Nb(C,N) precipitation during early rolling stages. In order to avoid this, one possibility is to add Mo and limit the amount of Nb. This study analyzes the influence of different Mo contents on static recrystallization of Nb microalloyed steels. The analysis includes the evaluation of the incidence of the combined effect of Nb and Mo solute drag retardation in both heterogeneous recrystallization evolution and grain size distribution for the case of initial coarse austenite grain sizes. To do that, partially recrystallized microstructures have been analyzed and microstructural parameters of interest quantified.

Softening Kinetics in High Al-Nb Microalloyed Steels

The combined effect of Al and Nb additions on the static softening behavior of C-Mn steels was investigated. The compositions of the steels studied in this work are representative of the recently developed TRIP-assisted steels: a base composition of 0.2wt.%C, 2wt.%Mn, 50ppmN, three different Al levels, 0.03 (base steel), 1 and 2wt.%, and two Nb contents of 0.03 and 0.07wt.%. Double-hit torsion tests were performed at different deformation temperatures (925-1065oC) and pass-strains (=0.2 and 0.35). It was found that solute Al produced a significant retardation on the static softening kinetics, this effect being enhanced by the addition of Nb. Additionally, below 1000oC the addition of 2 wt.%Al promotes the γα phase transformation to occur concurrently with softening. For the Nb microalloyed steels strain induced precipitation also occurred, resulting in a complex interaction between softening, phase transformation and strain induced precipitation.

Physical Modelling of the Interaction between Softening and Nb (C,N) Strain-Induced Precipitation in High-Mn Steels

The applicability of a physical model to describe the austenite microstructure evolution after hot deformation in High-Mn steels was investigated. Double-hit torsion tests were carried out to determine the softening behaviour of two High-Mn steels, one of them microalloyed with 0.11 wt% Nb. The values of the unknown parameters included in the model were determined by fitting experimental results. The model describes adequately the softening evolution of the steels. At high temperatures recovery and recrystallization contribute to mechanical softening, the latter having the main contribution. In contrast, when strain-induced precipitation occurs recovery has a larger effect.

Static Softening Behaviour in High Aluminum Steels

The effect of Al addition on the static softening behavior of C-Mn steels was investigated. The compositions of the steels studied are representative of the recently developed TRIP-assisted steels: a base composition of 0.2%C, 2%Mn, 50ppm N and three different Al levels, 0.03 (base steel), 1 and 2%. Double-hit torsion tests were performed at different deformation temperatures, in the range 950°C to 1100°C, and pass-strains, =0.2 and 0.35. It was found that solute Al produced a significant retardation on static recrystallization kinetics, equivalent to that exerted by 0.026%Nb for the 1%Al steel and to 0.05%Nb for the 2%Al steel. Additionally, at the lowest temperatures (950-1000°C) and 2%Al level, austenite to ferrite phase transformation was found to be concurrent with softening, enhancing retardation in the mechanical softening.