Microstructural and precipitation characterization in Nb-Mo microalloyed steels: Estimation of the contributions to the strength

The influence of coiling temperature on the final microstructure and precipitation has been analyzed in several low carbon Nb and Nb-Mo microalloyed steels. A throughout characterization of the complex microstructures has been performed using electron backscattered diffraction, measuring low and high angle unit sizes, microstructural substructure, as well as quantifying the homogeneity. An important microstructural refinement is observed for all compositions as the coiling temperature decreases. Regarding precipitation, the coiling temperature strongly modifies the size and density of the fine precipitates, being 550 °C the optimal coiling temperature for the Nb-Mo steels. The addition of Mo to Nb steels provides a refinement of the precipitates and, therefore, enhances their contribution to strengthening. Considering all the microstructural and precipitation quantification data, the yield strength was estimated and the contribution of the different mechanisms calculated. The grain size contribution is proven to be the most important factor regarding strengthening, followed by dislocation density and precipitation especially at low coiling temperatures and Nb-Mo steels.     

Precipitation Strengthening by Induction Treatment in High Strength Low Carbon Microalloyed Hot-Rolled Plates

Metallurgical and Materials Transactions A - The use of microalloyed steels in the production of thick plates is expanding due to the possibility of achieving attractive combinations of strength...     

Effect of Nb and Mo on Austenite Microstructural Evolution During Hot Deformation in Boron High Strength Steels

Metallurgical and Materials Transactions A - This work has focused on the study of hot working behavior of boron high strength steels microalloyed with different combinations of Nb and/or Mo. The...     

Microstructural Features Controlling Mechanical Properties in Nb-Mo Microalloyed Steels. Part II: Impact Toughness

The present paper is the final part of a two-part paper where the influence of coiling temperature on the final microstructure and mechanical properties of Nb-Mo microalloyed steels is described. More specifically, this second paper deals with the different mechanisms affecting impact toughness. A detailed microstructural characterization and the relations linking the microstructural parameters and the tensile properties have already been discussed in Part I. Using these results as a starting point, the present work takes a step forward and develops a methodology for consistently incorporating the effect of the microstructural heterogeneity into the existing relations that link the Charpy impact toughness to the microstructure. In conventional heat treatments or rolling schedules, the microstructure can be properly described by its mean attributes, and the ductile–brittle transition temperatures measured by Charpy tests can be properly predicted. However, when different microalloying elements are added and multiphase microstructures are formed, the influences of microstructural heterogeneity and secondary hard phases have to be included in a modified equation in order to accurately predict the DB transition temperature in Nb and Nb-Mo microalloyed steels.     

Effects of Platelet-Rich Plasma on Cellular Populations of the Central Nervous System: The Influence of Donor Age

Platelet-rich plasma (PRP) is a biologic therapy that promotes healing responses across multiple medical fields, including the central nervous system (CNS). The efficacy of this therapy depends on several factors such as the donor’s health status and age. This work aims to prove the effect of PRP on cellular models of the CNS, considering the differences between PRP from young and elderly donors. Two different PRP pools were prepared from donors 65–85 and 20–25 years old. The cellular and molecular composition of both PRPs were analyzed. Subsequently, the cellular response was evaluated in CNS in vitro models, studying proliferation, neurogenesis, synaptogenesis, and inflammation. While no differences in the cellular composition of PRPs were found, the molecular composition of the Young PRP showed lower levels of inflammatory molecules such as CCL-11, as well as the presence of other factors not found in Aged PRP (GDF-11). Although both PRPs had effects in terms of reducing neural progenitor cell apoptosis, stabilizing neuronal synapses, and decreasing inflammation in the microglia, the effect of the Young PRP was more pronounced. In conclusion, the molecular composition of the PRP, conditioned by the age of the donors, affects the magnitude of the biological response.     

Microstructural Features Controlling Mechanical Properties in Nb-Mo Microalloyed Steels. Part I: Yield Strength

Low carbon Nb-Mo microalloyed steels show interesting synergies between the “micro”-alloying elements when high strength–high toughness properties are required. Strain accumulation in austenite is enhanced, and therefore grain sizes are refined in the final microstructures. The presence of Mo facilitates the presence of non-polygonal phases, and this constituent modification induces an increment in strength through a substructure formation as well as through an increase in the dislocation density. Regarding fine precipitation and its strengthening effect, the mean size of NbC is reduced in the presence of Mo and their fraction increased, thus enhancing their contribution to yield strength. In this paper, a detailed characterization of the microstructural features of a series of microalloyed steels is described using the electron-backscattered diffraction technique. Mean crystallographic unit sizes, a grain boundary misorientation analysis, and dislocation density measurements are performed. Transmission electron microscopy is carried out to analyze the chemical composition of the precipitates and to estimate their volume fraction. In this first part, the contribution of different strengthening mechanisms to yield strength is evaluated and the calculated value is compared to tensile test results for different coiling temperatures and compositions.     

Effect of Composition and Deformation on Coarse-Grained Austenite Transformation in Nb-Mo Microalloyed Steels

Thermomechanical processing of microalloyed steels containing niobium can be performed to obtain deformed austenite prior to transformation. Accelerated cooling can be employed to refine the final microstructure and, consequently, to improve both strength and toughness. This general rule is fulfilled if the transformation occurs on a quite homogeneous austenite microstructure. Nevertheless, the presence of coarse austenite grains before transformation in different industrial processes is a usual source of concern, and regarding toughness, the coarsest high-angle boundary units would determine its final value. Sets of deformation dilatometry tests were carried out using three 0.06 pct Nb microalloyed steels to evaluate the effect of Mo alloying additions (0, 0.16, and 0.31 pct Mo) on final transformation from both recrystallized and unrecrystallized coarse-grained austenite. Continuous cooling transformation (CCT) diagrams were created, and detailed microstructural characterization was achieved through the use of optical microscopy (OM), field emission gun scanning electron microscopy (FEGSEM), and electron backscattered diffraction (EBSD). The resultant microstructures ranged from polygonal ferrite (PF) and pearlite (P) at slow cooling ranges to bainitic ferrite (BF) accompanied by martensite (M) for fast cooling rates. Plastic deformation of the parent austenite accelerated both ferrite and bainite transformation, moving the CCT curves to higher temperatures and shorter times. However, an increase in the final heterogeneity was observed when BF packets were formed, creating coarse high-angle grain boundary units.