Precipitation of Nb in Ferrite After Austenite Conditioning. Part I: Microstructural Characterization

Journal of Materials Processing Technology, 2006

Materials Science and Engineering: A, 2014

The microstructure-property relationship of an NbTi-microalloyed ferritic steel was studied as a function of thermo-mechanical schedule using a Gleeble 3500 simulator, optical and scanning electron microscope, and atom probe tomography. Contributions to the yield stress from grain size, solid solution, work hardening, particle and cluster strengthening were calculated using the established equations and the measured microstructural parameters. With a decrease in the austenite deformation temperature the yield stress decreased, following a decrease in the number density of 420 nm Nb-rich particles and E5 nm Nb-C clusters, although the grain refinement contribution increased. To achieve the maximum cluster/precipitation strengthening in ferrite, the austenite deformation should be carried out in the recrystallisation temperature region where there is a limited tendency for strain-induced precipitation. Based on the analysis of cluster strengthening increment, it could be suggested that the mechanism of dislocation-cluster interaction is closer to shearing than looping.

A Nb microalloyed steel has been thermomechanically processed at laboratory through the use of plane strain compression sequences followed by simulated coiling. Tensile samples have been machined from the obtained specimens in order to investigate the effect of different variables: recrystallisation or accumulated strain before transformation, holding in austenite and coiling temperature on the final mechanical behaviour. Transmission electron microscopy observation of the precipitates has been carried out after coiling at different temperatures. It has been shown that when Nb remains in solution in austenite after hot deformation, it can precipitate in ferrite, leading to an important strengthening effect which is directly related to the concentration of Nb in solution before transformation and coiling temperature.

Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2005

In recent years, several studies on grain refinement of ferrite have been conducted by different methods (equal channel angular pressing (ECAP), high pressure torsion (HPT), accumulative roll bonding (ARB)) in order to optimize the relationship between mechanical properties and microstructure of steels. The present work deals with the strain-induced dynamic transformation of ferrite. Samples of low C-Mn and Nb-Ti microalloyed steels were intensively deformed in warm torsion. This resulted in the formation of ultrafine ferrite grains (∼1 m) thereby enhancing the mechanical properties compared to the ordinary hot rolled products without the necessity of implementing an extremely high strain accumulation. The following annealing has resulted in recrystallization and growth of ferrite grains accompanied by a homogenization of the final microstructure and a decrease in hardness. With increase in annealing time the volume fraction of martensite/austenite (MA) constituent increased. The results have shown that the effect of steel chemistry was insignificant in comparison with the effect of thermomechanical processing parameters.

Metallurgical and Materials Transactions A, 2017

AMAIA IZA-MENDIA, DENIS JORGE-BADIOLA, and ISABEL GUTIÉ RREZ Very often Nb contributes to the strength of a microalloyed steel beyond the expected level due to the grain size strengthening resulting from thermomechanical processing. Two different mechanisms are behind this phenomenon, and both of them have to do with the amount of Nb remaining in the solution after hot rolling. The first of them is the increase of the hardenability of the steel due to Nb, and the second one is the fine precipitation of NbC in ferrite. The contribution of the precipitates to the work hardening of two thermally and thermomechanically processed microalloyed steels is addressed in this work and this contribution has been integrated into previously developed models by the authors for ferrite-pearlite microstructures. An L eff is considered through the effective spacing associated to the different obstacles and their interactions with the moving dislocations. The model obtained shows good agreement with the experimental tensile curves from the end of yield point elongation to the onset of necking.

Materials Science Forum

Three novel low carbon microalloyed steels with various additions of Mo, Nb and V were investigated after thermomechanical processing simulations designed to obtain ferrite-bainite microstructure. With the increase in microalloying element additions from the High V- to NbV- to MoNbV-microalloyed steel, the high temperature flow stresses increased. The MoNbV and NbV steels have shown a slightly higher non-recrystallization temperature (1000 °C) than the High V steel (975 °C) due to the solute drag from Nb and Mo atoms and austenite precipitation of Nb-rich particles. The ambient temperature microstructures of all steels consisted predominantly of polygonal ferrite with a small amount of granular bainite. Precipitation of Nb-and Mo-containing carbonitrides (>20 nm size) was observed in the MoNbV and NbV steels, whereas only coarser (~40 nm) iron carbides were present in the High V steel. Finer grain size and larger granular bainite fraction resulted in a higher hardness of MoNbV st...

Metallurgical and Materials Transactions A, 2002

This work presents an austenite decomposition model, based on the thermodynamics of the system and diffusion-controlled nucleation theory, to predict the evolution of microstructure during hot working of niobium-microalloyed steels. The differences in microstructural development of hotdeformed microalloyed steel in the single-phase austenite and two-phase (austenite ϩ ferrite) regions have been effectively described using an integrated computer modeling process. The complete model presented here takes into account the kinetics of recrystallization, recrystallized austenite grain size, precipitation, phase transformation, and the resulting ferrite structure. After considering existing austenite decomposition models, we decided that the method adopted in the present work relies on isothermal transformation kinetics and the principle-of-additivity rule. The thermomechanical part of the modeling process was carried out using the finite-element method. Experimental results at different temperatures, strain rates, and strain levels were obtained using a Gleeble thermomechanical simulator. A comparison of results of the model with experiments shows good agreement.

ISIJ International, 1998

The transformation behaviours and microstructural characteristics of three B-containing steels were investigated. [n particu lar, the effects of deformation in the no-recrystallization temperature range and cool ing rate were studied by means of compression tests. It was found that over a large cooling rate range (from 1 to 50'Cls). Mo-Nb-B steel exhibits microstructures consisting of a mixture of plate-like or lath-like ferrite with retained austenite or martensite (i.e. M/A) islands. This is basically a low carbon bainitic microstructure. and can be identified as B, in the Bramfitt and Speer classification system. The lengths of the ferrite laths increase and the widths decrease as the cooling rate is increased. The shapes and distributions of the M/A islands change from being blocky and randomly distributed to fine and more aligned, as the cooling rate is increased. Also, the lengths of the bainitic ferrite laths are shortened by heavy deformation in the norecrystallization temperature range. The microstructures of the Nb-1 5B and B-only steels are basically polygonal ferrite at low coo]ing rates, however, the fractions of bainite in these two grades increase with cooling rate. The minimum cooling rate required for avoiding polygonal ferrite formation during continuous cooling are much higher in these two grades than in the Mo-Nb-B steel.

AIP Conference Proceedings , 2019

In the present investigation, high carbon steel and Nb microalloyed steel have been subjected to hot rolling (~80% hot deformation) by varying the finish rolling temperature (FRT) followed by air cooling to room temperature. The specimens were austenitised at 1200°C for 1 h and then subjected to hot deformation with two FRTs of 800°C and 1000°C. The deformation at a higher temperature results in a continuous grain boundary network structure with apparent evidence of recrystallisation whereas, finer grains at lower deformation temperature has been observed. The volumetric percentage of ferrite has been found to be more in case of Nb microalloyed steel at a lower FRT whereas, more volume percentage of pearlite has been observed when cooled from higher deformation temperature. On the other hand, deformation at a lower temperature (800°C FRT) results in more amount of ferrite formation in the Nb microalloyed steel. The average hardness value has been found to be higher (≈270 HV30/20) for the Nb microalloyed steels at higher deformation temperature (1000°C FRT) which is attributed to the finer interlamellar spacing (≤ 100 nm). Subsequent air cooling to room temperature from a higher deformation temperature increases the ultimate tensile strength (UTS) of high carbon steel marginally but improve the UTS of Nb microalloyed steel significantly. The tensile fracture morphology reveals the abundant presence of dimples at a lower deformation temperature, indicating a ductile fracture. The fracture surface of the Nb microalloyed steel subjected to higher deformation temperature exhibits a typical river-like pattern indicating cleavage fracture. Finally, a correlation between microstructure and properties have been established.

Materials Science and Engineering: A, 2005

The microstructural evolution during hot rolling of a commercially developed hot rolled Nb-Ti steel with a yield strength of 770 MPa is described and analyzed in terms of strengthening mechanisms. The objective of the study is to examine the constituents of the microstructure (type of microstructure, nature of precipitates, dislocation density) that contributed to the attractive strength-toughness combination of a new high strength 770 MPa Nb-Ti microalloyed steel. From the transmission electron microscopy observations, the precipitates can be categorized into four classes depending on their size and shape. Type I were intergranular rod-like (Fe,Mn) 3 C precipitates, while type II were TiN precipitates of size range 120-500 nm containing small amounts of niobium. The type III precipitates identified as (Nb,Ti)C were ∼10-200 nm size and randomly distributed in the matrix, and type IV were spherical or needle-shaped (3-5 nm) (Nb,Ti)C precipitates that nucleated preferentially on sub-boundaries and dislocations in ferrite. The dislocation density was high in some grains and less in other grains. The high dislocation density and fine-scale precipitation are the dominant factors responsible for the high strength of 770 MPa microalloyed hot rolled steel.