Precipitate Morphology and Chemistry in Reheated and As-cast Nb-Ti Microalloyed Austenite (original) (raw)
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Metallurgical and Materials Transactions A, 2012
Austenite-grain growth was investigated in a couple of microalloyed steels, one containing Ti and the other containing Nb, Ti, and V, using different reheating temperatures between 1273 K and 1523 K (1000°C and 1250°C). Nature and distribution of microalloy precipitates were quantitatively analyzed before and after reheating. Interdendritic segregation (or microsegregation) during casting can result in an inhomogeneous distribution of microalloy precipitates in the as-cast slabs, which can create austenite grain size variation (even grain size bimodality) after reheating. Ti addition reduced the grain size variation; however, it could not eliminate the grain size bimodality in Nb-containing steel, due to the differential pinning effect of Nb precipitates. A model was proposed for the prediction of austenite grain size variation in reheated steel by combining different models on microsegregation during solidification, thermodynamic stability, and dissolution of microalloy precipitates and austenite grain growth during reheating.
SEM CHARACTERIZATION OF COMPLEX PRECIPITATES IN AN AS-CAST Nb-V-Ti-MICROALLOYED STEEL
This article deals with the characterization of complex precipitates from commercial Nb-V-Ti microalloyed steel produced by continuous casting, using fracture surfaces from V-notch Charpy tests, and extracted precipitates. The samples were studied by means of scanning electron microscopy and X-ray energy dispersive spectroscopy, in combination with X-ray diffraction. Samples examination revealed micron size (Nb,Ti,V)(C,N) precipitates, most of them with a highly branched dendritic morphology, and Ti rich precipitates, corresponding to (Ti,Nb,V) (C , N). Also dendritic Nb(C,N) without Ti was observed. By means of the method used in this investigation, it was possible to easily reveal the precipitates, being able to fully characterize their distribution, size and morphology, and also their chemical composition and crystal structure.
Dissolution Behavior of NB-Rich Precipitates During Reheating of Microalloyed Steel Heavy Plates
Tecnologia em Metalurgia Materiais e Mineração
The effect of reheating temperature of slabs on the dissolution of precipitates and mechanical properties of Ti-Nb microalloyed steel was investigated in pilot scale. Transmission electron microscopy indicated a significant decrease in the presence of precipitates formed during the solidification of the steel as the reheating temperature increased from 1100°C to 1200°C. With further temperature increase up to 1280°C no significant changes in size and shape of the precipitates were observed. Together with the precipitates dissolution behavior, an increase in strength was observed with the elevation of the reheating temperature up to 1200°C, due in part to the reprecipitation of fine Nb-rich precipitates during hot rolling. When reheating at 1200°C the presence of deformed ferrite on the microstructure contributed to the additional increase in strength. After reheating above 1200°C up to 1280°C, a decrease in strength of the rolled plates was observed. This effect has been credited to the absence of deformed ferrite grains and to a small increase in the low angle grain boundaries size. The results were in agreement with Thermo-Calc and Dictra simulations.
Materials Science and Technology, 1997
A commercial microalloyed steel was examined by optical and transmission electron microscopy in order to determine the origin and the amount of carbonitride precipitation which had formed as the result of industrial processing on a hot strip mill. Almost one-half of the total microalloy addition (0'06Ti and 0'02Nb) had remained in the form of eutectic or undissolved particles after soaking. Intragranular nucleation of fine carbonitride particles was observed to have occurred only in austenite. No additional carbonitride particles had formed in ferrite during coiling and almost one-half of the microalloying elements dissolved during soaking had remained in solution at the end of the industrial rolling process. According to well established structure-property relationships, a precipitation strengthening potential of60-80 MN m-2 could be assigned to thecarbonitride particles which had nucleated in austenite. MSTj3416
Influence of Ti and N Contents on Austenite Grain Control and Precipitate Size in Structural Steels
Isij International, 1999
The austenite grain size of six steels with different Ti and N contents has been determined at 1 300 degrees C x 10 min and at 1 100 degrees C x 10 min. The most stable grain was achieved for a Ti/N ratio close to 2. In parallel, a study of precipitate sizes was carried out using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results indicate that the steels with a lower N content and approximately the same Ti content show a distribution of precipitates of larger size, which increases with the Ti content in solution at the austenitization temperature applied. As the austenitization temperature increases, the double effect of precipitate dissolution and coarsening appears, and only with a hipostoichiometric Ti/N ratio and as precipitated Ti content above 0.010 (wt%) are they capable of controlling the austenite grain up to high temperatures.
Precipitate Composition and Morphology During Reheating of Low Carbon Microalloyed Steel for CCR
The precipitates present in a controlled rolled, low carbon, Ti-Nb microalloyed steel have been studied on reheating to 850-1400°C, Below 1_100°C, (Nbrich'Ti) spheres dissolve and coarsen, and spherlcal caps and nodules of (Nbrich'Ti) appear on the faces and corners of (Tiric ,Nb) cubes and laths. AlN coprecipitates with many of t£e caps and cubes. At higher temperatures than 1100°C, Nbrich spheres and caps dissolve completely, leaving only Tirich cubes and laths , which are stable to 1400°C. Characteristically, these are compositionally cored and laths are richer than the cubes.
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 1996
This work describes the effect of Nb supersaturation in austenite, as it applies to the strain-induced precipitation potential of Nb(CN), on the suppression of the static recrystallization of austenite during an isothermal holding period following deformation. Four low carbon steels, microalloyed with Nb, were used in this investigation. Three of the steels had variations in Nb levels at constant C and N concentrations. Two steels had different N levels at constant C and Nb concentrations. The results from the isothermal deformation experiments and the subsequent measurement of the solution behavior of Nb in austenite show that the recrystallization-stop temperature (T RXN) increases with increasing Nb supersaturation in austenite. Quantitative transmission electron microscopy analysis revealed that the volume fraction of Nb(CN) at austenite grain boundaries or subgrain boundaries was 1.5 to 2 times larger than Nb(CN) volume fractions found within the grain interiors. This high, localized volume fraction of Nb(CN) subsequently led to high values for the precipitate pinning force (F PIN). These values forF PIN were much higher than what would have been predicted from equilibrium thermodynamics describing the solution behavior of Nb in austenite.
Materials Science and Engineering: A, 2013
The effect of deformation temperature on Nb solute clustering, precipitation and the kinetics of austenite recrystallisation were studied in a steel containing 0.081C-0.021Ti-0.064 Nb (wt%). Thermo-mechanical processing was carried out using a Gleeble 3500 simulator. The austenite microstructure was studied using a combination of optical microscopy, transmission electron microscopy, and atom probe microscopy, enabling a careful characterisation of grain size, as well as Nb-rich clustering and precipitation processes. A correlation between the austenite recrystallisation kinetics and the chemistry, size and number density of Nb-rich solute atom clusters, and NbTi(C,N) precipitates was established via the austenite deformation temperature. Specifically, we have determined thresholds for the onset of recrystallisation: for deformation levels above 75% and temperatures above 825 °C, Nb atom clusters recrystallisation
Metallurgical and Materials Transactions A, 2018
The solidification path of a high-temperature processed (HTP) X65 sour service steel with 0.039 wt pct C, 0.09 wt pct Nb, and 0.54 wt pct Mn and its effect on the segregation, microstructure, and precipitation distribution of Ti,Nb(C,N) was studied using optical and confocal microscopy, scanning electron microscopy (SEM), and computational simulation (Thermo-Calc and DICTRA). The results were compared with those obtained for another commercial microalloyed steel, containing 0.09 wt pct C, 0.04 wt pct Nb, and 0.97 wt pct Mn. The results indicate that the main parameter that influences microsegregation is the C content, which has a large influence on the solidification path. The difference in segregation between different positions in industrial continuous cast slabs of the steels was also observed, as expected. The larger solidification interval (T L-T S) of the commercial microalloyed steel indicates the formation of a solidification front that has higher solute concentration than the X65 HTP sour service steel, which concurs with the higher macro-and microsegregation observed.
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 1994
This work describes the effect of temperature on both the microstructure and composition of microalloyed steel austenite in the as-reheated condition. Four laboratory steels of similar C levels were analyzed in this investigation. Three steels had different Nb concentrations at con-stant N levels, and the fourth exhibited a difference in N concentration. The average prior-austenite grain size was determined using quantitative metallographic techniques as a function of reheat temperature. The corresponding amount of Nb in solution in austenite was determined from atom probe analysis. Results from this investigation indicate that at elevated temperatures, representative of typical reheating practice, a smaller amount of Nb is soluble in austenite than what would have been predicted from any existing solubility relation for NbCx in austenite. The solubility of Nb and C in austenite for the low-N steels is described by the relation: Log [Nb][C] = 2.06 - 6700/T. Additionally, it is shown that undissolved NbCx particles are present in austenite approximately 125 ‡C above the grain-coarsening temperature.