Nb Solute Drag Research Articles

Effect of Nb on Microstructural Evolution and Mechanical Properties of Hot‐Rolled Quenching and Partitioning Steels Containing Bainite

Herein, the effect of Nb content on the phase transformation kinetics, microstructure, and mechanical properties of hot‐rolled quenching and partitioning (Q&P) steel is investigated. The characteristics of three C–Mn–Si–Ti steels (0.18C, 2.0Si, 2.6Mn, and 0.015Ti) containing 0, 0.027, or 0.061 wt% Nb are compared. Results reveal that grain boundary pinning by precipitates and Nb solute drag effects refine the austenite grain size during the hot‐rolling process; the microstructural refinement is carried over to the final microstructure subjected to the Q&P treatment. The remaining supersaturated Nb suppresses the bainite formation and decreases the final bainite fraction formed in the Q&P process. The microstructural evolution leads to an increase in the ultimate tensile strength (UTS) of the steel containing 0.027 wt% Nb from 1169 to 1228 MPa, while keeping the total elongation at 18%. When the Nb content is increased to 0.061 wt%, the UTS of the steel increases to 1313 MPa, but the elongation at break drops to 16%. The effect is due to the carbon consumption by the Nb precipitates, which causes a decrease in the stability of the retained austenite and reduces the strain hardening at high strain levels.

Effect of different welding energy on microstructure and toughness of HAZ of low carbon bainitic steel

Low carbon bainitic steel (LCBS) with excellent combined properties is the first choice for materials of pipeline transiting geological disaster. However, welding will worsen its toughness. In this paper, three kinds of welding heat input were designed to study the relationship between the toughness and microstructure in the coarse grain zone of welding heat affect zone (CGHAZ) of LCBS. The evolution characteristics of the microstructure of LCBS and the CGHAZ, and impact fracture were investigated by optical microscopy (OM), scanning electron microscopy (SEM). The results indicated that microstructure of LCBS consists mainly of bainite ferrite (BF) and granular bainite (GB). Heat input for 22 kJ/cm, the original austenite grains become coarsening, the microstructure is a small amount of quasi polygonal ferrite (QF) and polygonal ferrite (PF), which exhibits low Charpy impact toughness. However, for heat input of 19 kJ/cm, the degree of grain coarsening is small and distribution of martensite-austenite (M-A) constituents is the chain. The statistics of image software show that with the increase of heat input (16–22 kJ/cm), the average grain size of original austenite is basically the same (25 um), which is mainly due to Nb solute drag restraint the growth of austenite grain.

Assessing the recovery and recrystallization kinetics of cold rolled microalloyed steel through coercive field measurements

Annealing after cold rolling brings about the activation of recovery and recrystallization in microalloyed steels. The importance of recovery has been most often associated with its effects on the recrystallization kinetics. However, recovery has gained particular importance as an alternative heat treatment under the name “back annealing” or “recovery annealing”. In the present work, various annealing treatments were applied to a Nb-microalloyed steel in the range of temperatures and times where recrystallization is not complete. As a consequence, a large set of tensile strength-ductility pairs was obtained, even for conditions in which recrystallization was avoided. Through non-destructive magnetic coercive field measurements, recovery and partial recrystallization were monitored for each annealing treatment. Magnetic softening is significantly greater than mechanical softening. The variation in recovery in terms of temperature and time is highly affected by the presence of Nb in solution in the hot band (before cold rolling). At low recovery annealing temperatures, 350–450°C, Nb solute drag on dislocations is the main mechanism that controls recovery, whereas at 550°C, Nb strain induced precipitation leads to a recovery plateau in terms of coercive field.

The Effect of Solute Nb on the Austenite-to-Ferrite Transformation

Niobium is a widely used micro-alloying element in steels that can retard the austenite-to-ferrite transformation primarily by solute drag when Nb remains in solution. It is critical to develop quantitative models to predict the effect of Nb on the transformation kinetics. In the present work, dedicated continuous cooling transformation (CCT) studies were performed for a low-carbon steel microalloyed with 0.047 wt pct Nb starting from fully recrystallized austenite states with the same grain size but different amounts of Nb in solution. The austenite-to-ferrite transformation kinetics is described from a fundamental perspective by assuming a mixed-mode reaction including solute drag of Nb. Using the solute drag model of Fazeli and Militzer, the intrinsic interface mobility, trans-interface diffusivity of Nb, and its binding energy to the interface have been determined from the CCT data. The interfacial parameters are critically analyzed and compared with independent measurements of diffusion and grain boundary segregation.

Multipass hot deformation behaviour of high Al and Al–Nb steels

The effect of Nb (up to 0.07wt%) and high Al content (up to 2wt%) on the multipass deformation behaviour of steels with 0.2% C and 2% Mn was studied with the aid of hot torsion simulations. From the tests, the critical Non-Recrystallisation (Tnr), Recrystallisation Limit and Stop Temperatures (RLT and RST) and the ferrite phase transformation start temperature (Ar3) were determined. It was observed that an increase in Al content from 1% to 2% or a microalloying addition of 0.03% Nb to 1% Al steel both led to a significant increase in the recrystallisation critical temperatures, which is greater than 100°C in the case of the Tnr. However, the value of the Tnr was not affected when 0.03% or 0.07% Nb was added to the 2% Al steel. Specimens quenched after several deformation passes were examined by optical and TEM means in order to study the interaction between static recrystallisation, strain-induced precipitation and γ→α phase transformation, and determine the mechanisms leading to strain accumulation in the steels investigated. The results suggest that for the 1% Al steels, the Al and Nb solute drag effect is the main mechanism leading to the increase in the critical recrystallisation temperatures, while for the 2% Al steels the occurrence of γ→α phase transformation at temperatures close to the Tnr is the main mechanism involved in softening retardation, with a limited contribution of Nb. However, γ→α phase transformation taking place at temperatures close to the Tnr resulted in a loss of hot ductility, which can limit the industrial applicability of the 2% Al steels.

Austenite grain growth prediction coupling with drag and pinning effects in low carbon Nb microalloyed steels

A metallurgical model has been developed to predict the austenite grain growth in Nb microalloyed steels. The mutual effects of Nb(CN) particle pinning and Nb solute drag on grain growth kinetics are studied. The particle dissolution, the undissolved particle coarsening and the changes in Nb solute in solution during reheating or isothermal heat treatment process are taken into account in the model. It is shown that, besides the pinning exerted by the NbC precipitates, the solute drag of Nb in solid solution plays an important role in the inhibition of austenite grain growth in Nb microalloyed steels. The Nb solute drag effect on grain growth decreases with increasing temperature because the grain boundary can gradually break away from the solute atmosphere in the higher velocity region at high temperature. The mean austenite grain size sluggishly increases with temperature in the low temperature region, while it significantly increases in the relative high temperature region. The predicted austenite grain size concerning the combined effect of Nb drag and Nb(CN) pinning is in good agreement with the experimental results from the literature.

EFFECT OF Nb SOLUTE DRAG AND NbC PRECIPITATE PINNING ON THE RECRYSTALLIZATION GRAIN GROWTH IN LOW CARBON Nb-MICROACLOYED STEEL

A kinetic equation for austenite grain growth has been derived concerning the mutual effect of NbC and Nb solute in low carbon Nb-microalloyed steels. It is shown that both solute drag of Nb in solid solution and pinning of NbC particles inhibit the grain boundary migration during grain growth after recrystallization in low carbon Nb-microalloyed steels. At high temperatures the NbC pinning plays a dominate role for retarding the austenite grain growth with less Nb solute drag effect. An obvious Nb solute drag restraint was, however, observed at relatively low temperatures. Also, the theoretical calculations are in good agreement with experimental results. The effectiveness of drag effect of soluble atoms and pinning effect of precipitates can be characterized by a p factor. The pinning of precipitates and solute drag of soluble atoms are more effective for suppressing grain growth as p> 0a nd p< 0, respectively. And the ratio of Nb in solute and Nb in precipitate as p=0 reaches the priority and most effectively retards the grain growth. In the traditional hot rolling or austenitizing temperature range, a strong suppression for grain growth after recrystallization could be obtained due to those fine NbC particles smaller than 10 nm in Nb-microalloyed steels.

The Onset Temperatures of γ to α-Phase Transformation in Hot Deformed and Non-deformed Nb Micro-alloyed Steels

In the present paper, continuous cooling transformation behaviors in Nb micro-alloyed steels were systematically investigated by the thermal dilatation method, during which the effects of Nb contents and hot deformation of austenite on phase transformation behaviors, especially the transformation start temperatures, were studied in detail. The tests were carried out with the samples having been reheated+hot deformed or reheated+non-deformed prior to the dilation measurements. It was found that the Ar3 temperatures measured from the dilatometric curves changed with increasing Nb content in parabolic ways under both hot deformed and non-deformed conditions. It is believed that dissolved Nb in austenite may have had the Nb solute drag effect that could delay austenite to ferrite transformation. On the other hand, Nb precipitates in austenite could retard the growth of austenite grains and act as potential nucleation sites, both of which could enhance the transformation kinetics. Also, the interaction of strain, precipitation and temperature was estimated by using the Sellars model, which predicted that the strain induced precipitation had occurred in hot deformed Nb steels before phase transformation started, and in non-deformed steels with Nb content greater than 0.023 mass%, precipitation was also likely to have occurred under slow cooling rate before phase transformation started, which could have played an important role in determining Ar3. These factors worked together to make Ar3 changing with Nb content in the parabolic way. Based on the experimental results, a mathematical model for the Ar3 calculation for Nb and C–Mn steels were developed, which exhibited a good accuracy in predicting the Ar3 of the steels with and without hot deformation.

Effect of Nb on grain growth of ferrite in C-Mn steel during isothermal holding after severe deformation

The effect of Nb on grain growth of ferrite during isothermal holding after severe deformation was investigated. For Nb and C-Mn steels, the deformation was carried out in the temperature range 645-772°C, encompassing ferrite transformation temperature Ar3, after reheating at 900°C and 1250°C. After reheating at 900°C, the strain induced ferrite grains with an average grain size of 2 μm readily formed up to the equilibrium fraction at all deformation temperatures. Grain growth of the strain induced ferrite in the Nb steel was significantly inhibited during the following isothermal holding because of the grain boundary pinning by NbC precipitates. After reheating at 1250°C, the strain induced ferrite transformation was apparently reduced by solute drag of Nb so that strain free ferrite grains were formed during the subsequent isothermal holding. When strain free ferrite formed near the deformed ferrite grains, abnormal grain growth by strain induced boundary migration (SIBM) occurred in the Nb steel, as well as in the C-Mn steel. The SIBM occurred owing to the stored energy difference between the deformed ferrite and the strain free ferrite regardless of the presence of fine NbC precipitates or partially recrystallised grains.

Effect of Sintering Atmosphere on Grain Boundary Segregation and Grain Growth in Niobium‐Doped SrTiO3

To investigate the donor segregation in grain boundary regions and its effect on grain growth in SrTiO3, SrTiO3 powder compacts were doped with Nb2O5 and sintered in air or in hydrogen. The Nb‐doped SrTiO3 sintered in air did not show any detectable Nb segregation at the grain boundary region while an appreciable segregation was observed in the space‐charge region of the sample sintered in H2. The observed donor segregation in H2 suggests a negative grain boundary charge and compensating positive space charge, which are the opposite to those in air. The negative grain boundary core was attributed to the segregation of inherently present acceptor impurities and the trapping of electrons at grain boundaries. In the H2‐sintered sample, where the added Nb ions were segregated in the space‐charge region, the grain growth was suppressed. This result may indicate that the grain growth suppression in H2 is due to the Nb solute drag of the boundary motion and the reduction in Ti vacancies.

Flow stress modeling and warm rolling simulation behavior of two Ti–Nb interstitial-free steels in the ferrite region

Axisymmetric compression tests and compact strip production (CSP) warm rolling simulations were carried out on two commercial Ti–Nb stabilized interstitial-free (IF) steels to investigate their flow behavior in the ferrite region. The comparison between the measured and predicted stress curves indicated that dynamic recovery was the dominant softening mechanism during axisymmetric compression deformation. The flow stress curves during CSP-warm rolling simulations indicated that static and dynamic recovery dominated the softening process for early passes and an apparent dynamic recrystallization contributed to the flow stress reductions for the last two passes. The microstructural evolution during the simulation confirmed that the very fine quasi-equiaxed ferrite grains obtained could be explained by apparent dynamic recrystallization, which was assisted by dynamic recovery at high temperature and a reduced Nb solute drag.