Nb-containing Steel Research Articles

Impact of niobium addition and non-metallic inclusions’ characteristics on the microstructure and mechanical properties of low-carbon CrNiMnMoB ultrahigh-strength steel: A comprehensive investigation

The effect of cooling rate and Nb addition on the phase transformation, microstructures, and mechanical properties of thermomechanically controlled hot-rolled low-carbon CrNiMnMoB ultrahigh-strength steel were studied. The aspects impeding Nb's benefits on hardenability, phase transformation, and mechanical properties were investigated. The hot rolling process was simulated, and a continuous cooling transformation diagram was constructed for the studied steels. Microstructures were analysed using laser scanning confocal microscopy and field emission scanning electron microscopy, with the latter also used for in-depth investigations of the non-metallic inclusions. Transmission electron microscopy was employed to investigate the precipitate characteristics. The concentrations of Nb in the solution and precipitates were quantified for the martensitic and granular bainitic structures. The effect of Nb in solution on phase transformation and strengthening is diminished, and the hardness is reduced by Nb precipitation at an early processing stage. Decreasing the cooling rate promotes the formation of granular bainitic structures rather than martensite. After being subjected to hot rolling followed by water quenching and air cooling, the microstructures are autotempered martensite and fully granular bainite, respectively. Various non-metallic inclusions, including Al2O3, MnS, and Al2O3–MnS, MnS·BN, Al2O3·BN and MnS·Al2O3·BN were observed. The number density and area fraction of non-metallic inclusions in the Nb-containing steel and the air-cooled samples are higher than those observed in the Nb-free and water-quenched samples. Adding 0.04 wt% of Nb slightly increased the strength, but the toughness properties dropped, mainly due to the higher number density and area fraction of non-metallic inclusions and primary large precipitates.

The Performance of Niobium-Microalloying Ultra-High-Strength Bridge Cable Steel during Hot Rolling.

This study focuses on exploring the effects of niobium (Nb)-microalloying on the properties of steel for ultra-high-strength bridge cables during hot-rolling processes. We employed a combination of dual-pass compression tests, stress-strain curve analysis, and Electron Backscatter Diffraction (EBSD) techniques to investigate the influence of Nb-microalloying on the static recrystallization behavior and grain size of the steel. The key findings reveal that Nb-microalloying effectively inhibits static recrystallization, particularly at higher temperatures, significantly reducing the volume fraction of recrystallized grains, resulting in a finer grain size and enhanced deformation resistance. Secondly, at a deformation temperature of 975 °C, Nb-containing steel exhibited finer grain sizes compared to Nb-free steel when held for 10 to 50 s; however, the grain size growth accelerated when the hold time exceeded 50 s, likely linked to the increased deformation resistance induced by Nb. Lastly, this research proposes optimal hot-rolling process parameters for new bridge cable steel, recommending specific finishing rolling temperatures and inter-pass times for both Nb-containing and Nb-free steels during the roughing and finishing stages. This study suggests optimal hot-rolling parameters for both Nb-containing and Nb-free steels, providing essential insights for improving hot-rolling and microalloying processes in high-carbon steels for bridge cables.

Strengthening of high nitrogen austenitic stainless steel by Nb addition

Nitrogen addition is known to be one of the most effective ways of strengthening austenitic stainless steels while maintaining good properties of the steels. This study investigates Nb addition as an additional strategy to strengthen a high nitrogen austenitic stainless steel. It was found that Nb-containing steels developed Z-phase precipitates during homogenization and hot-rolling. Their fraction and size hardly changed by subsequent aging at 1050 °C. They effectively delayed grain coarsening and fine grains were maintained throughout the aging. The yield strengths of Nb-containing steels were always higher than those of Nb-free steel by 30–83 MPa. Quantitative analyses revealed that the major strengthening mechanism was grain boundary strengthening. The contribution from precipitation hardening was small because the precipitates were not sufficiently fine and the negative contribution from solid solution strengthening due to N consumption by the precipitation was also insignificant.

Effect of Nb-induced microstructure on pipeline steel corrosion and stress corrosion cracking performance in acidic environment

The corrosion and stress corrosion cracking response of two steels were examined in acidic environment. The surface microstructure and texture of the steels were significantly affected by a one-order-of-magnitude difference in Nb content. The Nb-containing steel had a refined microstructure, with more {110}-oriented grains, whereas the Nb-deficient steel had an irregular microstructure, containing both fine and large polygonal ferrite grains, with a strong {001}<110> component. When compared to the Nb-containing steel, the Nb-deficient steel had lower corrosion resistance, more delamination cracks, and a higher tendency for embrittlement. This disparity in performance was clearly related to the microstructural characteristics of the steels.

Effects of Cu and Nb on martensite hardening and transformation-induced plastic deformation behavior of maraging TRIP-aided steel

Martensite hardening and transformation-induced plastic deformation (TRIP) are crucial to the mechanical properties of steels. In this study, maraging TRIP-aided steel was developed. The addition of Cu and Nb can effectively promote transformation-induced plastic deformation, this is due to the characteristic of the alternating combination of soft and hard phases. In contrast, the addition of Nb has a more significant effect on martensite hardening, which can be attributed to the smaller effect of Nb on the martensite transformation start temperature (Ms). The multi-phase structure and precipitation strengthening, lead to the ultimate tensile strength and elongation of the maraging TRIP-aided steel is ∼1300 MPa and ∼24%, respectively. It was found that the prior austenite formed a continuous network surrounding bainite and martensite. The high-density dislocations in bainite and martensite increased the hardness. In addition, the metastable austenite phase was transformed into the martensite phase, and a heterostructure with the alternating distribution of bainite/austenite was formed during deformation. The refinement of grains effectively reduced the concentration of stress. Massive nanometer-scaled precipitates were formed in Nb-containing steel after peak aging, and the co-precipitation of Nb-rich and Mo-rich phases occurred in the steel. The large-sized Laves-Fe2(Nb, Mo) effectively enhanced the strength of steel, and the fine and dense Ni3Ti was beneficial to the simultaneous improvement of strength and plasticity. Therefore, the jointed mechanisms of martensite hardening, and the TRIP effect improved the strength while retaining good plasticity.

Effect of Nb Content on High-Temperature Strength and Precipitates of Nb-Containing Steel

Reasonable control of the content of niobium (Nb) in a steel slab is of great significance to improve the strength of the slab and the regularity of precipitation of precipitates. In this study, the high-temperature strength and precipitates of the same steel with four different Nb contents (A: 0.006%, B: 0.031%, C: 0.050%, D: 0.065%) were tested and analyzed. The results show that when the Nb content is 0.031%, the tensile strength and yield strength of the steel reach the ideal state; the two-phase particles that precipitated in the Q355GJ steel after heat treatment are mainly square- and star-shaped (Ti, Nb) (C, N) composite precipitates, and their amount increases with the increase in Nb content. Most of the two-phase particles (84%) precipitated in the steel with the Nb content of 0.031% are smaller than 80 nm, and the continuous increase in the Nb content cannot increase the precipitation amount of the two-phase particles in the steel, but can increase the size of the two-phase particles.

Investigation of Strain-Induced Precipitation of Niobium Carbide in Niobium Micro-Alloyed Steels at Elevated Temperatures

Two steels with a base composition of Fe-0.2C-0.8Mn-1.2Cr (wt%) but with different niobium (Nb) contents (0.02 and 0.03 wt%) were employed to study the effect of precipitate evolution on the softening resistance in the austenite region under elevated temperature deformation. The thermomechanical procedure was executed by a deformation-dilatometer and involved double deformation processes with 25% strain at a 0.25 s−1 strain rate at 900, 925, 950, and 1000 °C. The softening ratios, reflecting the competition between recrystallization and precipitation, were evaluated. The results indicated that both steels showed better softening resistance at 900 °C than at other temperatures. However, the softening ratio of 0.03 wt% Nb-containing steel (Steel 3N) rose after 100 s at 900 °C, while 0.02 wt% Nb-containing steel (Steel 2N) maintained a low softening ratio within 300 s at 900 °C, indicating that Steel 3N was relatively non-durable. A microstructural characterization showed that in the Steel 3N sample deformed at 900 °C, recrystallization occurred more strongly than for Steel 2N after a 1000 s holding time. A follow-up analysis then showed that Steel 3N treated at 900 °C revealed a faster coarsening of the carbides than Steel 2N even in the early stage of precipitation, evidencing that Steel 2N exhibited a lower softening resistance at 900 °C.

Austenite Grain Growth Law of High-strength Steel for Offshore Engineering

The metallographic analysis method is used to study the original austenite grain change law of Nb-containing high-strength ship steel. Adding Nb to the experimental steel will refine the grains to a certain extent. Different Nb content, the corresponding grain size change trend is different. Under the experimental heating environment of 850°C~1150°C, the grains of the two experimental steels have different degrees of coarsening at different heating temperatures. Compared with the low-C and high-Nb steel (EH36) with the high-C and low-Nb steel (AH36), the former can not only control the austenite grain growth well at lower temperature, but also has a strong inhibitory effect in a higher temperature environment. Based on the experimental data, the austenite grain growth kinetic model of the two experimental steels at the heating temperature of 850°C~1150°C is obtained.

Effect of Slab Reheating Temperature on Cold Rolling Texture Evolution of Nb-Containing Grain-Oriented Silicon Steel

Texture control of grain-oriented silicon steel is the key factor to ensure the magnetic properties of the finished product. Nb-containing grain-oriented silicon steel with different slab reheating temperatures was hot rolled followed by single-stage or two-stage cold rolling, and the textures were also analyzed. In the single-stage cold rolling process, as the slab reheating temperature is reduced, the intensity of the rotating cube texture {100}&lt;011&gt; and Goss texture {011}&lt;100&gt; drops, and that of the {111}&lt;112&gt; texture increases. In the two-stage cold rolling process, with the decrease in the slab reheating temperature, the intensity of the {111}&lt;112&gt; texture increases from 4.958 to 6.809. At the same slab reheating temperature, the intensity of the rotating cube texture declines more significantly in the two-stage cold rolling process. Finally, two-stage cold rolling with the slab reheating temperature of 1220 °C is found to be more beneficial for the formation of a sharp Goss texture during the second recrystallization. The magnetic induction intensity B800 of the final product is 1.87 T, and the iron loss P1.7/50 is 1.36 W/kg.

The effect of Ti addition on the wear resistance of Nb-containing cast steel

ABSTRACT In this study, the dry sliding friction and wear tests of low-carbon microalloyed steel containing Nb and Ti were performed by using MFT-5000 universal friction and wear tester. An analysis was conducted on the effect of Ti on the wear behaviour and mechanism of experimental steel containing Nb. The morphology exhibited by the worn surface and the microstructure of the plastic deformation layer was characterised under scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). In addition, the phase distribution in the experimental steel was identified with electron back scatter diffraction (EBSD). As indicated from the experimentally achieved results, the wear rates of the two types of experimental steel samples were upregulated with the increase in the load and the rotational speed; with the addition of Ti element, the chain segregation of NbC could be obviously facilitated, the grain uniformity could be promoted, and the chain NbC could be prevented from cracking and damaging the matrix in the wear process. Moreover, with the introduction of Ti element, the strength and hardness exhibited by the matrix were improved, the oxide layer adhered to the matrix better, and the wear mechanism varied from abrasive-adhesive wear to oxidation-adhesive wear, thereby effectively improving the wear resistance of the experimental steel.

Precipitate Evolution in 22Cr25NiWCuCo(Nb) Austenitic Heat-Resistant Stainless Steel during Heat Treatment at 1200 °C.

In this study, 22Cr25NiWCuCo(Nb) heat-resistant steel specimens with high Cr and Ni contents were adopted to investigate the effect of Nb content on thermal and precipitation behavior. Differential scanning calorimetry profiles revealed that the melting point of the 22Cr25NiWCuCo(Nb) steel specimens decreased slightly with the Nb content. After heat treatment at 1200 °C for 2 h, the precipitates dissolved in a Nb-free steel matrix. In addition, the Z phase (CrNb(C, N)) and MX (Nb(C, N), (Cr, Fe)(C, N), and NbC) could be observed in the Nb-containing steel specimens. The amount and volume fraction of the precipitates increased with the Nb content, and the precipitates were distributed heterogeneously along the grain boundary and inside the grain. Even when the heat treatment duration was extended to 6 h, the austenitic grain size and precipitates became coarser; the volume fraction of the precipitates also increased at 1200 °C. The Z phase, rather than the MX phase, became the dominant precipitates at this temperature.

Microstructure evolution and corrosion behavior of Nb-alloyed cast heat-resistant steel during different aging treatments

The microstructure evolution of heat-resistant cast stainless steels aged at 600, 700, 800 and 900 °C for 2 h was investigated by scanning electron microscopy and X-ray diffraction. Potentiodynamic polarization curves were applied to study the effects of Nb addition and aging temperature on corrosion resistance. The results demonstrated that eutectoid decomposition of the ferritic phase (δ → σ + γ2) was observed at 700–900 °C. The content of σ-phase first increased and then decreased in the steels, where the maximum content was obtained at 800 °C, indicating that the highest hardness occurred at 800 °C accordingly. The hardness of Nb-containing steels was significantly higher than that of Nb-free steels. The corrosion resistance of heat-resistant cast steels in various aging temperatures was different due to the formation of σ-phase. Both Cr-rich carbides and σ-phase were harmful to the corrosion resistance, while Cr-rich carbides were the main factor. Nb-containing heat-resistant cast steels exhibited superior corrosion resistance, as Cr-rich carbides were reduced and the corrosion products of Nb-rich slowed down the formation of steady-state pits.

Grain growth during reheating of HSLA steels with a narrow segregation separation

Reheat treatments for two HSLA steels, a forged Al-Nb-containing steel (Steel-1) and an as-rolled Nb-containing steel (Steel-2), were carried out for one to three hours at temperatures between 1090 and 1250°C which were selected by the precipitates’ dissolution temperatures predicted by Thermo-Calc and DICTRA. Steel-1 contains continuous segregation bands with segregation spacing of 65 ± 10 µm, whereas Steel-2 contains discontinuous segregation bands with spacing of 35 ± 10 µm. Uniform grain size distributions occurred in Steel-1 for all reheat temperatures whereas abnormally large grains were observed in Steel-2 when reheating at 1090°C. A mechanism for grain size development in materials with continuous and discontinuous segregation bands based on austenite nucleation and growth and precipitate pinning in the segregation band is proposed.

Effect of Nb Addition on Oxidation Mechanisms of High Cr Ferritic Steel in Ar–H2–H2O

High chromium ferritic steels are being used as construction materials for interconnects in solid oxide electrolysis cells (SOEC). Addition of niobium in the range of a few tenths of a percent is suitable for increasing the high-temperature creep strength of this type of ferritic steel. In the present work, the high-temperature isothermal oxidation behavior of a niobium containing ferritic steel at 800 °C was investigated in Ar–4%H2–4%H2O gas simulating the service environment in an SOEC (cathode side) and compared with that of a Nb-free counterpart alloy. Gravimetric data were correlated with the results from microstructural analyses using, among others, scanning and transmission electron microscopy as well as glow discharge optical emission spectroscopy. Atom probe tomography was used for obtaining atomic-scale insight into the segregation processes in external oxides and their interfaces. The oxidation rate was substantially higher for the Nb-containing than for the Nb-free alloy. Both alloys formed double-layered oxide scales consisting of inner chromia and outer MnCr2O4 spinel. Additionally, a thin layer of rutile-type Nb(Ti,Cr)O2 oxide of 200–300 nm thickness was observed at the scale–alloy interface in the Nb-containing steel. Nb addition to the alloy led to its segregation at chromia grain boundaries which affected the diffusion of Cr and other solute species such as Ti, Mn and Si.

Effect of Nb and Cu on the microstructure and hardness of quenching–partitioning–tempering dual-phase steels

ABSTRACTThe effect of Nb and Cu on the microstructure and hardness of quenching–partitioning–tempering (Q–P–T) dual-phase steels was investigated. Nanostructured martensite and retained austenite were obtained when the steels were treated with a Q–P–T process. The hardness, volume fraction of retained austenite and final carbon content in austenite of the Nb + Cu-containing steel were higher than the Nb-containing steel. This was attributed to the promotion of Nb precipitation by the addition of Cu.

Effect of Niobium on the Secondary Precipitates and Tempering Resistance of Spray-Formed M3:2 High-Speed Steel

The precipitation of secondary carbides in spray-formed M3:2 high-speed steels with or without 2% niobium (Nb) addition during overtempering at 600 °C was characterized, and the carbides were identified in detail using transmission electron microscopy. Strengthening in the peak-hardened state is due to coherent precipitates of types MC and M2C. During overtempering, the hardness of M3:2 steel greatly decreases due to M2C coarsening with the precipitation of M3C carbide, whereas the very fine and dense nature of the M2C precipitates is responsible for the remarkable long-time strength exhibited by the Nb-containing steel. The MC precipitates, however, are fairly stable. The addition of Nb results in substantial dissolution of Mo and Cr into the matrix prior to tempering, which refines the secondary M2C precipitate dispersion and delays overaging reactions; therefore, superior tempering resistance can be obtained.

Fatigue life of aged Nb-containing ferritic stainless steel JFE429EX welded joint in laboratory air and 3%NaCl solution

Fatigue life of aged Nb-containing ferritic stainless steel JFE429EX welded joint in laboratory air and 3%NaCl solution

Effect of niobium on microstructure and precipitation in As-annealed Sanicro 25 steel

ABSTRACT The chemical compositions of Sanicro 25 stainless steel added with different concentrations of niobium (Nb) (0, 0.29, 0.58, and 0.86 wt-%) were investigated. X-ray diffraction analysis showed that the matrix of all the prepared materials was austenite. The average grain size decreased from 426 to 90 µm with the increase in Nb content. Electron backscatter diffraction and energy dispersive X-ray spectroscopy analyses showed that Cr23C6 was the main precipitate in the Nb-free steel, whereas Nb(C, N) were the main precipitates in Nb-containing steel. The increase in Nb content also increased the volume fraction and average particle size of the precipitates.

Effect of molybdenum on phase transformation and microstructural evolution of strip cast steels containing niobium

Molybdenum (Mo) is known to have a complex effect on phase transformations and precipitation in steels manufactured by conventional casting. The present work aims to examine the effect of Mo on phase transformations in Nb-containing steels produced by strip casting. Advanced experimental techniques have been utilised to simulate the strip casting process, and the microstructural features of the rapid solidification are retained for further study. Two cooling conditions from the austenite phase field were examined, isothermal holding and continuous cooling. It was found that at high cooling rates, the addition of Mo delayed the nucleation of bainite and lowered the bainite start temperature, but did not alter the bainite growth rate. The addition of Mo was also found to result in a slower transformation rate of polygonal ferrite under both isothermal and continuous cooling conditions. Thermodynamic simulations indicated that Mo did not affect the growth velocity of the polygonal ferrite, and quantitative metallography showed the nucleation density was significantly reduced by Mo addition. For the slowest continuous cooling rate, the addition of Mo completely inhibited pearlite formation, with bainitic ferrite forming instead. This has been suggested to be the result of the suppression of pearlite nucleation, rather than inhibition of growth.

Grain growth behaviour on reheating Al–Nb-containing steel in the homogenised condition

ABSTRACTGrain size development during reheating is important to the mechanical properties of steels, and non-uniform grain growth including abnormal and bimodal grain growth are undesirable for steel manufacturing. In this paper, reheating treatments for Al–Nb-containing steel (0.057 wt-% Al and 0.019 wt-% Nb) were selected based on Thermo-Calc predictions for the dissolution temperatures of precipitates in the steel for a fully homogenised condition. Abnormal grain growth occurred in the homogenised Al–Nb sample during reheating, associated with random dissolution of the grain boundary-pinning AlN precipitates, while bimodal grain growth was not observed. So material in a homogenised condition would not be expected to provide a fully uniform grain structure during reheating treatment.