Nb Microalloying Research Articles

Hardenability of Medium Carbon Cr-Mn-Mo Alloyed Steels

Three medium carbon Cr-Mn-Mo structural steels with different content of alloying elements were studied. The austenite transformation during continuous cooling was investigated using dilatometer and metallographic analysis. The CCT diagrams were plotted showing the effect of the increased alloying elements content and B and Nb micro-alloying on the hardenability of the studied steels. The hardness dependences on the cooling rate were obtained.

Challenges of Nb Application in Thermomechanical Processes of Steels for Long Products

Nb is a classical microalloying element in the design of thermomechanical treatments in low carbon steels for flat products applications. However, its use in medium-high carbon grades, as occurs in hot rolling of bars, is less common. This is, in part, because of the diversity of characteristics required to those grades of steels and the less knowledge about the function of Nb in these cases. Consequently, less information is reported concerning thermo-mechanical processing of Nb microalloyed steels in long products applications. In this case, it is necessary to consider the singularities related to these processes, such as the short interpass times and the wide range of chemical compositions usually applied on these products. Short interpass times result in high strain rate values that can lead to metallurgical changes on the mechanisms occurring during the hot rolling must be considered. Moreover, the high Carbon contents applied in long products, usually between 0.20–0.40%, can influence the Nb solubility and precipitation in each stage of the process: prior to hot rolling on the reheating furnace, during the process and after hot rolling, depending on the cooling strategy adopted and on the post-rolling heat treatments that can be applied. This paper analyses different singularities associated with the use of Nb microalloying for long products. Several aspects, such as the partial or complete dissolution of the Nb prior to hot rolling, its role in the control of austenite microstructure and its incidence in the final microstructure and mechanical properties, will be considered.

Microstructure-Tensile Properties Relationship and Austenite Stability of a Nb-Mo Micro-Alloyed Medium-Mn TRIP Steel

This study investigated the microstructure–tensile properties relationship and the retained austenite room temperature stability of a Nb and Mo micro-alloyed medium manganese transformation induced plasticity (TRIP) steel. A number of findings were obtained. Most importantly, the steel after being processed by quenching and tempering (Q & T) exhibited excellent tensile properties, i.e., the strength of 878–1373 MPa, the ductility of 18–40% Mo, and Nb microalloying served to control the fraction of retained austenite and to improve tensile strength by fine grain strengthening. Excellent tensile properties were attributable to the large amount of retained austenite which produced a discontinuous TRIP effect. This effect led to the production a large amount of martensite which relieved the stress concentration, contributing to the coordinated deformation between the phases and thus improving the deformability of the steel. Additionally, the differences in Mn and C contents led to varying degrees of austenite stability and the length of the Lüders band decreased as the intercritical annealing temperature increased. The micro-alloyed medium manganese steel experimented on our study showed considerable improvement in tensile properties in comparison with the 5Mn-0.1C medium manganese steel in previous studies.

Effect of Nb on the hydrogen-induced cracking of high-strength low-alloy steel

Hydrogen-induced cracking (HIC) susceptibility tests, hydrogen permeation tests and the electron backscatter diffraction (EBSD) measurements were used to study the effect of Nb on the HIC resistance of the high-strength low-alloy (HSLA) steel. The Nb microalloying significantly improved the HIC resistance and decreased the proportion of intergranular cracks through the hydrogen trapping effect and microstructure optimization. Dispersed nanometer-sized NbC precipitates effectively increased the number of irreversible traps, hindered hydrogen aggregation, and enhanced the resistance to crack initiation and propagation. Moreover, Nb optimized the microstructure by increasing the proportion of the low-angle grain boundaries (LAGBs), refining the prior austenite grain size, decreasing the dislocation density, and changing the distribution of Σ3 boundaries.

Effect of Nb micro-alloying on microstructure and properties of thermo-mechanically processed high carbon pearlitic steel

Two C-Mn-Si steels with and without Nb micro-alloying are selected in the present study. Both these high carbon steels have been thermo-mechanically processed using Gleeble 3800 simulator. Optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), atomic force microscope (AFM) were utilised for microstructural characterisation. The austenitising temperature was varied from 1150 °C to 1200 °C followed by hot compression and subjected to free cooling to evaluate the influence of austenite grain size on transformed microstructure and mechanical properties. It is evident that higher austenite grain size is obtained for both types of steels subjected to higher austenitising temperature. The refinement of pearlite interlamellar spacing (<0.20 μm) with degenerated morphology as well as finer prior austenite grain size (<40 μm) is found to be more prominent for Nb micro-alloyed sample. The average hardness values of both the steels are higher for the specimens, treated at a higher austenitising temperature which is attributed to finer interlamellar spacing. Finally, the correlation between the evolving microstructure and resulting mechanical properties (hardness and yield strength) has been made.

The Effect of Nb Micro-alloying on the Bainitic Phase Transformation Under Strip Casting Conditions

The effect of Nb concentration on the transformation from austenite to bainitic ferrite has been examined under simulated strip casting conditions. Nb concentration was found to delay the nucleation of bainite, but accelerated its growth. It is suggested that the delay in nucleation increases the driving force for transformation, which results in an increase in the growth rate of the bainite. The bainite/austenite interfaces are proposed to move too quickly to suffer appreciable solute drag.

Development of hot stamping grade steel with improved impact toughness by Nb microalloying

Low carbon alloy steels, one alloyed with B and Cr and other alloyed with B, Cr and Nb were made and cast into pencil ingots. These ingots were hot-rolled to 2 mm sheets suitable for hot stamping application in an experimental rolling mill and a detailed study was made on the effect of various alloying elements on the microstructure and mechanical properties of these steels. A significantly higher impact toughness (42 J) could be achieved in the as-rolled and water-quenched Nb-Cr-B steel in comparison to Cr-B steel under similar processing condition. Microstructural refinement of martensite must be the reason for achieving high impact toughness.

Effects of Niobium on the Mechanical Properties and Corrosion Behavior of Simulated Weld HAZ of HSLA Steel

Simulated microstructures of the TZ, ICHAZ, FGHAZ, and CGHAZ of weld joints made from two kinds of HSLA steels with 0 or 0.079 wt pct Nb were prepared by means of heat treatment. Optical microscopy and transmission electron microscopy were used to observe microstructures and the distribution of nanosized precipitates in the simulated weld heat-affected zone (HAZ). Mechanical properties of the simulated HAZ were measured by tensile tests, and the corrosion behavior in simulated seawater was studied using electrochemical and immersion tests. It was shown that the ICHAZ and CGHAZ possess the worst overall mechanical properties in both kinds of HSLA steels, and the corrosion resistance in the descending order was the BM, TZ, FGHAZ, ICHAZ, and CGHAZ. Contrasting Nb-bearing and Nb-free steel demonstrated that the strength and corrosion resistance of the simulated HAZ were enhanced by Nb microalloying, which resulted in precipitation, homogeneous microstructures, and other relative sequences. Moreover, the surface of the Nb-bearing steel formed compact corrosion product films with higher resistance to ion migration; thus, the initiation and propagation of pitting holes were effectively inhibited.

Partially-recrystallized, Nb-alloyed TWIP steels with a superior strength-ductility balance

We investigated the effect of Nb micro-alloying in the range of 0.01 to 0.l wt% on the microstructures and mechanical properties of Fe17Mn0.6C1.5Al (wt%) TWIP steel. EBSD analysis shows that the Nb addition retards recrystallization in both the hot-rolled steels and cold-rolled and annealed steels. The Nb addition in the cold-rolled and annealed TWIP steel leads to an increase in yield strength. This phenomenon is due to a combined effect of precipitation hardening and a low degree of recrystallization. Recovery annealing of the cold-rolled TWIP steels at 650°C results in a good combination of yield strength and ductility. The steels containing 0.01wt% and 0.025wt% of Nb show a superior combination of UTS×TE exceeding 40,000MPa∙% and yield strength higher than 800MPa. The design of TWIP steels utilizing both precipitation hardening and partial recrystallization opens a way to develop steels with a superior combination of yield strength and ductility.

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.

Nb-Microalloying in Next-Generation Flat-Rolled Steels: An Overview

Extensive efforts are underway worldwide to develop new steels with substantial fractions of retained austenite, for lightweight automobile manufacturing and other applications requiring improved combinations of strength and formability. It is likely that microalloying can provide product enhancements in these emerging products, such as Q&amp;P, TBF, medium-Mn TRIP, etc. and this paper examines the expected behavior of niobium using inferences based on published AHSS literature and principles of Nb microalloying. Some benefits of Nb in terms of microstructure refinement and precipitation strengthening have been reported. The potential influences of Nb are complex due to the sensitivity of Nb dissolution and precipitation to chemical composition and processing; differences in the expected role of Nb are pointed out with respect to different product forms produced via hot-rolling or annealing after cold-rolling, and microstructures with or without substantial quantities of primary ferrite. Some issues that warrant further examination are identified, as a deep understanding of Nb microalloying and other fundamental behaviors will be needed to optimize the performance of these next-generation steels.

Determination of the Grain Coarsening Temperature in Nb Microalloyed Steels by Multiphase-Field Model

The grain coarsening temperature in Nb microalloyed steels is investigated by multiphase-field model. In this study, the pinning force is treated as time-dependent using mean-field kinetics of precipitates including volume fraction and their size. The grain size is calculated with time under various temperature range. The grain coarsening temperature is determined by the ratio of the largest radius of grain vs. the average grain radius criteria (Rmax / Ravg &gt; 2.94) in two-dimensional growth. Through this model, it is possible to simulate grain growth in microalloyed steels more precisely.

Effect of Coiling Temperature on the Microstructure and Mechanical Properties of Hot-Rolled Ti–Nb Microalloyed Ultra High Strength Steel

A novel low carbon Ti–Nb microalloyed hot rolled steel with minimum yield strength of 700 MPa and good balance of stretch-flangeability and impact toughness has been developed by controlled thermo-mechanical processing following thin slab direct rolling route. In the present work, the effects of two coiling temperatures on the resulting microstructure, micro-texture and mechanical properties on this Ti–Nb microalloyed steel have been studied. It is observed that increase in coiling temperature from 520 to 580 °C significantly affects the mechanical properties. Higher dislocation density and increased precipitation along with slightly smaller grain size is observed for 580 °C coiling temperature resulting in about 50 MPa increase in yield and tensile strengths as compared to 520 °C coiling temperature.

Effect of Nb microalloying on the heat affected zone microstructure of girth welded joints

The effect of Nb content in the range 0.07–0.10% on the HAZ microstructure of large diameter pipes is reported in this paper. Results show that Nb is able to influence the size of the bainitic packet and cells in the heat affected zone, i.e. the microstructural parameters affecting impact toughness and hardness behavior. Such an effect leads to an improvement of both toughness and hardness for the steel with higher Nb content.

Development of a high strength and ductile Nb-bearing dual phase steel by cold-rolling and intercritical annealing of the ferrite-martensite microstructures

Dual phase (DP) steels containing different amounts of Nb microalloying element were produced by cold-rolling followed by intercritical annealing of a ferrite-martensite duplex starting structure. The effects of Nb contents (0.00, 0.06, 0.12 and 0.18wt%) and intercritical annealing time on the microstructural evolutions and mechanical properties were studied. Results from microscopic images showed that increasing Nb content increased the volume fraction of martensite and decreased the average grain size of ferrite. Tensile results illustrated an excellent strength-elongation balance in terms of energy absorption (160Jcm−3) and toughness (229MPa). The lowest grain size of about 1.40±0.37µm was achieved in the DP steel containing 0.12wt% Nb whose strength was about 123% higher than that of the as-received ferritic-pearlitic steel (e.g. 540MPa), without loss of ductility. The variations of strength, elongation and fracture mechanism of the specimens with Nb contents and intercritical annealing time were correlated to the microstructural features.

Influence of Nb Micro-alloying on TRIP Steels Treated by Continuous Cooling Process

TRIP (transformation induced plasticity) steels are low alloyed steels with multiphase microstructure consisting of ferrite, carbide-free bainite and retained austenite. They are typically produced by thermo-mechanical treatment, which involves the hold in bainite transformation region. The hold ensures enough bainite in the final microstructure and also helps to stabilize higher amount of retained austenite. Due to transformation induce plasticity effect; TRIP steels possess very good combination of high strength and high ductility. In response to industrial demands, C-Mn-Si and C-Mn-Si-Nb TRIP steels were subjected to thermo-mechanical treatment with continuous cooling which corresponded to real rolling mill processing of the steel with similar chemical compositions. Typical TRIP microstructures with 10-15% of retained austenite were achieved for both steels after optimization of cooling schedules. However, cooling by two different cooling rates had to be applied to C-Mn-Si steel to obtain the convenient microstructure. Beneficial effect of Nb micro-alloying on low sensitivity of TRIP steel to variations in cooling parameters has been found out. Mechanical properties of the most convenient microstructures were very promising, ultimate tensile strength reached 850MPa with ductility A5mm around 25%.

Modeling the hot flow behavior of a Fe–22Mn–0.41C–1.6Al–1.4Si TWIP steel microalloyed with Ti, V and Nb

The present research work analyses the influence of Ti, V and Nb microalloying elements on the hot flow behavior of a high-Mn Twinning Induced Plasticity (TWIP) steel. For this purpose, flow curves were obtained by uniaxial hot compression tests performed at four strain rates (10−1, 10−2, 10−3 and 10−4s−1) and three temperatures (900, 1000 and 1100°C). The models of Estrin, Mecking and Bergstrom; Avrami and Tegart, and Sellars were applied to determine the hot working constants used to derive the constitutive equations describing the flow curves. The analysis of modeling parameters of the hot flow curves shows that Ti, V and Nb additions to TWIP steel generated slight increase in the peak stress (σp), retardation of the dynamic recrystallization (DRX) onset, particularly at low temperature, and decrease in the activation energy required to recrystallization (Qt). Likewise, the softening effect promoted by DRV and DRX was more evident at high temperatures and low strain rates. On the other hand, the resulting deformed microstructures, analyzed by the SEM-EBSD technique, showed that the most important refining effect on recrystallized austenitic grain was in the presence of V and Ti. The good agreement between the experimental and predicted hot flow curves demonstrated that the developed constitutive equations predict with reasonable accuracy the hot flow behavior of the studied TWIP steels.

Study of Transformation Characteristics and Nano-Scale Precipitation in Nb Microalloyed Steel

In order to control the microstructure and nanoscale Nb(C, N) precipitation in a Nb microalloyed steel, three deformation schedules were employed, followed by cooling process to study the transformation and precipitation behaviors of the tested steel. It was found that plastic deformation in the non-recrystallization region of austenite accelerates the onset of transformation from austenite to ferrite, while the progress of ferrite transformation is retarded. The deformation also refines the ferrite grain size. The transformation behavior for deformation temperature of 850 and 910 °C are almost the same due to the similar deformed austenite. Although the latter deformation temperature is 60 °C higher, the deformation can also significantly refine the transformation microstructure. With the decrease in deformation temperature and increase in cooling rate, the precipitation start temperature decreases, and the nucleation zone of precipitation is transformed from austenite to ferrite or bainite under continuous cooling condition. When the cooling rate reaches 10 °C s−1, the nucleation of precipitation is totally inhibited during cooling process. The precipitate size is significantly refined by increasing cooling rate due to the increase of precipitation driving force.

Prior austenite grain size and tempering effects on the dislocation density of low-C Nb–Ti microalloyed lath martensite

The dislocation density of two as-quenched and quenched and tempered low-C Nb–Ti microalloyed martensitic steels was measured with X-ray diffraction for a range of prior austenite grain sizes. The dislocation density decreases with increasing prior austenite grain size in the as-quenched condition but the opposite occurs after high temperature tempering. The flow strength of all conditions is a function of dislocation density and follows a Taylor hardening model. The properties are insensitive to Nb microalloying for the two alloys assessed.

Softening Kinetics in High Al and High Al-Nb-Microalloyed Steels

Double-hit torsion tests were performed in order to study the effect of high Al levels (up to 2 wt.%) and Nb microalloying (up to 0.07 wt.%) on the static softening kinetics of 0.2%C-2%Mn steels. The addition of 1%Al leads to a delay in the softening kinetics due to solute-drag effect, equivalent to that exerted by 0.027%Nb. For the 2%Al steels, at temperatures below 1000 °C, γ → α phase transformation occurs after deformation, resulting in a larger retardation of the softening kinetics. At temperatures higher than 1000 °C, Nb in solid solution also contributes to the retardation of the static softening kinetics, and at lower temperatures NbC strain-induced precipitation leads to incomplete softening for the 1%Al steel, and to a complex interaction between softening, phase transformation, and NbC strain-induced precipitation for the 2%Al-Nb steels. The effect of Al on the static softening kinetics was quantified and introduced in a model developed in previous works for the prediction of the austenite microstructural evolution. In order to validate the results of the model, multipass torsion tests were carried out at conditions representative of hot strip and plate rolling mills. Model predictions show reasonable agreement with the results obtained at different deformation conditions.