Nb Precipitation Research Articles

Effect of Strain Rate on Nb(C,N) Precipitation in Micro Alloy Steel Slab

It was analyzed by strain-induced precipitation model that Nb(C,N) precipitation in micro alloy steel slab was effected by strain rate during continuous casting process. The results are as follows: The changing of casting speed could effect the time for 5%precipitation of Nb(C,N), which was decreasing with increasing casting speed at certain temperature and strain rate. Slab strain and strain rate were too small in bending zone and leveling zone. The effect of slab strain rate on Nb(C,N) precipitation could be ignore when Nb(C,N) precipitation in continuous casting process was studied.

Strain-induced precipitation and softening behaviors of high Nb microalloyed steels

The effects of Nb on strain-induced precipitation and softening were quantified by means of the Gleeble-3500 thermo-mechanical simulator and inductively coupled plasma-atomic emission spectrometry (ICP-AES). The results show that Nb cannot completely dissolve during the reheating processes in the high Nb steels. The undissolved precipitates act as heterogenous nucleation sites for the precipitates induced by the strain, and prolong the strain-induced precipitation start time shown in the stress relaxation curves. The strain-induced precipitation start time is linearly related to the content of supersaturated Nb (the difference between dissolved Nb content after soaking and dissolved Nb content in equilibrium at corresponding deformation temperature). Increasing the content of supersaturated Nb can shift the nose of the precipitation C-curve to shorter time and higher temperatures. The effect of content of supersaturated Nb on precipitation increases along with the decrease of temperature. The softening ratio is linear with dissolved Nb content before the strain-induced precipitation and with precipitable Nb content after the strain-induced precipitation. The precipitates induced by strain have more strong retarding effect on the static recrystallization than the solid solution of Nb atoms. However, with the increase of the average precipitate radius, the retarding effect of precipitate rapidly decreases.

On the Effect of Strain Reversal on Static Recrystallisation and Strain-Induced Precipitation Process Kinetics in Microalloyed Steels

Recent observations show that the strain reversal affects significantly and in a complex way both the static recrystallisation (SRX) and strain-induced precipitation (PPT) kinetics in Nb-microalloyed steel. It is already known that the recrystallisation stagnation is a consequence of the competition between the driving pressure for recrystallisation and the pinning pressure caused by the strain-induced precipitation of Nb (C,N) precipitates. Both of these parameters depend in turn on the local dislocation density. Thus, it is expected that a variation of the local dislocation density due to reversal of the strain will affect at the same time the local driving and the pinning pressures, which will cause the difference in the hardening levels. In the present paper, the influence of strain path change on microstructure evolution and mechanical behaviour in Nb-microalloyed steel (API X-70 grade) was studied. The deformation schedules were designed in order to investigate an effect of strain reversal on both static recrystallisation and strain-induced precipitation process kinetics. Flow curves recorded during deformation of X-70 steel showed clear influence of applied strain path on both static recrystallisation kinetics and strain-induced precipitation process.

Thermodynamic Calculation of Microalloy Precipitation in Austenite Region for Fe-C-N-Al-Ti-Nb Steel

A thermodynamic model of the microalloy precipitation elements in austenite region for Fe-C-N-Al-Ti-Nb steel has been established. Through calculating the action of the microalloying element in austenite, it can be found that carbon in steel accelerates the precipitation of Nb greatly, Nb in solution decreases quickly and the precipitation of Nb increases quickly with the increase of carbon. When the C in steel is low, the main constituent of the precipitation is TiN in high temperature, and TiN guadually turns to be NbC with the decrease of tempertature. When the carbon in steel is high，the main constituent in the precipitation is NbC even though the temperature is higher. The nitrogen in steel has a slight effect on NbN precipitation.

Study on Behavior of Carbonitride in Nb Micro-Alloyed Steel in CC-HR Region

As for the techno -interface of CC-HR region, the precipitation and dissolution of carbonitride in Nb micro-alloyed steel E36 were studied during continuous casting, roller conveying, stack cooling, heat-preserved and heating units in typical slab process flow by hot simulation experiment based on study of heat transfer. The results are as follows: Ti(C,N), and (Ti,Nb)(C,N) precipitate in turn in continuous casting and conveying. A few (Ti,Nb)(C,N) particles are not solid solute completely after heating. The precipitation of carbonitride is effected significantly by different cooling rate in conveying. When slab conveied at the rate of 6.00 °C/min and then cooled down to 600°C, the distribution density of carbonitride particles is higher, and the average equivalent diameter of carbonitride particles is smaller, some carbonitride particles distribute along the austenitic grain boundaries and the generation elements of carbonitride particles are under over-saturation state. When slab conveyed at the rate of 0.30 °C/min or 0.15 °C/min and then cooled down to 400°C, the distribution density of carbonitride particles is lower, and the average equivalent diameter of carbonitride particles is bigger, the carbonitride particles precipitate fully. The behavior of carbonitride in slab are effected by different thermal history in heating. The status of carbonitride is effected insignificantly by different thermal history after heating.

Strengthening from Nb-rich clusters in a Nb-microalloyed steel

We demonstrate that a Nb-microalloyed ultra-thin cast strip steel can be strengthened substantially without compromising ductility by performing a simple heat treatment at 700 °C for 4 min. The strengthening was attributed to a fine dispersion of Nb-rich solute atom clusters. These clusters had an average size of ∼60 atoms at peak hardness and resembled Guinier–Preston zones in Al–Cu alloys. The application of the Ashby–Orowan equation indicates that these clusters are potent strengthening agents when compared to conventional Nb(C,N) precipitation strengthening.

A Study on Cu and Nb Precipitation during High Temperature Tempering in Low Carbon Steels

The microstructure evolution and precipitation behavior of two low carbon steels are studied, with 0.05C-0.77%Nb added in one steel and (0.03C-)1.63Cu-0.74%Nb added in the other as a comparison. In the Cu-Nb steel tempered at 600°C for 18 hrs, there are two peaks in the particle size distribution figure, one between 2-3nm formed by NbCN precipitates, and the other, 10-12nm for Cu precipitates. The TEM observation on carbon replica shows that the average particle diameter of NbCN precipitate is 2.81±0.78nm in C-Nb steel, while 4.23±0.95 nm in Cu-Nb steel with lower carbon. The analysis shows that this size increase of NbCN not only decreases the precipitation strengthening, but also weakens significantly the pinning effect on the dislocations, which results in a more serious microstructure softening in Cu-Nb steel.

Investigation on Dissolving and Strain Induced Precipitation Behavior in Nb-Bearing Microalloyed Steel

Thermal simulation test, TEM(Transmission Electron Microscope) and nanobeam EDS techniques were used to investigate the dissolving and precipitation behavior in Nb-bearing mciroalloyed steel. The experimental results indicate that: there are two families of precipitates in the as forged samples. The larger precipitates of Nb(C,N) disappear after being held for 2h at 1300°C,while the inclusions of MnS formed from the solidification even remain for 48h held at that temperature. After 30% of predeformtion at 850°C and 900°C, the strain-induced precipitation occurs, that is, the other tiny precipitates. Compared with the samples relaxed at 850°C, the ones that are held at 900°C show larger particles by same holding time but demonstrate the same size when the relaxation time reaches 1000s.

Softening Kinetics in Nb-Microalloyed TRIP Steels with Increased Mn Content

Two 5Mn-1.5Al TRIP steels with and without Nb microaddition were developed in the present study. The steels contain bainite, martensite, interlath retained austenite and martensite- austenite islands. The paper presents the results of the compression tests carried out at various temperatures using the Gleeble simulator. To analyze the kinetics of static recrystallization in these steels, a softening kinetics were determined in a double-hit compression test. It was found that the dynamic recovery is a main thermally activated process occurring during hot deformation. The Nb microalloyed steel has higher flow stresses and peak strains than the Nb-free steel. A solute drag effect of Nb results in a slower softening kinetics of Nb containing steel. The effects of Mn on the retardation of Nb(C,N) precipitation and hot deformation characteristics are also discussed.

Carbide precipitation kinetics in austenite of a Nb–Ti–V microalloyed steel

The isothermal precipitation kinetics of carbides in both strain-free and strained austenite (γ) of a microalloyed steel were quantitatively investigated through the electrical resistivity and transmission electron microscopy. The (Nb,Ti)C carbides at the interfaces of the undissolved (Ti,Nb)(C,N) carbonitrides were observed at all temperatures in strain-free γ. However, for strain-induced precipitation, above 950 °C, the precipitation of (Nb,Ti)C carbides near the undissolved (Ti,Nb)(C,N) carbonitrides was predominant due to the recrystallization of strained γ. Meanwhile, the fine (Nb,Ti,V)C carbides were homogeneously precipitated in non-recrystallized γ at 850 °C and 900 °C, as well as near the undissolved (Ti,Nb)(C,N) particles. The electrical resistivity method was successfully used to quantitatively measure the isothermal precipitation kinetics of carbides in both strain-free and strained γ. The precipitation-time–temperature diagrams of the carbide in strain-free and strained γ, with nose temperatures of 950 °C, were generated and the precipitation kinetics were greatly accelerated by the applied strain.

Silicon as grain refiner in niobium microalloyed hot rolled steel

The effect of silicon on grain refinement has been investigated in industrially produced Nb microalloyed steel. The synergistic effect of silicon in the presence of Nb is found to favour grain refinement in hot rolled strip, which has resulted in very fine ferrite grains of <3; μm. Evolution of such a fine grain has been related to the enhanced precipitation of Nb(CN) in the presence of silicon. The concept of effective niobium (Nb + Si) in such steels has been introduced.

Study on the Production of Ti Micro-Alloyed High Strength Hot Rolled Steel by CSP Process

Ti micro-alloyed high strength hot rolled steel was developed in Valin Lianyuan Steel in the CSP line. The cleanliness of liquid steel was good enough for thin slab casting after LF refining. The mould powder was adjusted for stabilizing the heat flux of thin slab continuous casting mould. Homogeneous microstructure consisting of ferrite and pearlite was obtained in the hot rolled steel plates by the improving of rolling process. The nano-scale precipitates of Ti(C, N) and Nb(C, N) is the main strengthening mechanism. The yield strength of developed hot rolled plate is higher than 660 MPa and the tension strength is 760 MPa. The ductile-brittle transition temperature is below -60 degree Celsius. The developed Ti alloyed steel with designed composition fulfils the requirements of 600 MPa grade steel for engineering machinery.

Ultrahigh strength and low yield ratio of niobium-microalloyed 900 MPa pipeline steel with nano/ultrafine bainitic lath

An ultra-low carbon niobium-microalloyed steel with yield strength of ∼900 MPa has been processed on a pilot-plant scale. The microstructure of the steel is primarily characterized by lower bainite and acicular ferrite, with small fraction of lath-martensite and martensite–austenite (MA) constituents. Bainite is present as fine domains. A combination of niobium and titanium precipitates was observed at the grain boundaries and in the interior of the grains and includes irregular (∼40–150 nm of (Nb, Ti)(C, N)) and fine cuboidal/spherical particles of NbC (∼30–50 nm). It was observed that accelerated cooling inhibited the precipitation of Nb and Ti carbides. The Charpy impact toughness at −20 °C was 200 J and tensile elongation was 15% with the yield ratio of less than 0.84. The good matching of high strength and low yield ratio was realized by two-stage thermo-mechanical rolling combined with fast cooling.

Effect of Nb + Ti coating on the wetting behavior, interfacial microstructure, and mechanical properties of Al/Al2O3 joints

The subject of the work was to study the effect of Nb + Ti thin film deposited by PVD method on alumina substrates on the wetting behavior, bond strength properties, and structure of interface in the Al/Al2O3 joints. Applying the sessile drop method, the wetting behavior of molten Al (99,999%) on coated alumina substrates was studied in the temperature range between 953 and 1373 K under a vacuum of 0.2 mPa for 30 min of contact. The sessile drop samples were used to examine the interface structure, shear strength, and interfacial fracture toughness under the concentrated load. The introduction of the thin Nb + Ti film layer of 900 nm thickness: (1) greatly improves the wettability of alumina by molten Al at above 1223 K and the shear strength of Al/Al2O3 joints produced at 1223 K, (2) has positive effect on structure transformation in the interface and leads to fabrication of reliable metal–ceramic joints. Microstructural investigations of the interface indicated that the precipitates of Nb and Ti-rich intermetallic phases were formed at the Al/Al2O3 interface, which influenced strengthening of these joints. Hence a conclusion can be drawn that the interface structure influences the durability increase in Al/Al2O3 joints.

Refinement of CFB/M High Strength Steel Micro-Alloyed with Nb

A novel carbide free bainite/martensite (CFB/M) high strength steel of 1500MPa grade is micro-alloyed with Nb in order to refine the microstructure and improve the toughness. After the steel containing Nb was forged, coarse microstructure and strong transmissibility of structure were found. It is suggested that the large numbers of precipitates of Nb(CN) in the steel makes the structure transmissibility more serious. After solution treatment at 1200°C, both complete annealing twice and isothermal treatment at 600°C twice can eliminate the structure transmissibility and refine the structure. Investigation on refinement of cycle phase transformation shows that excellent effect of the grain refinement is obtained when the heating velocity exceeds some threshold. In addition, the temperature of austenitizing is of little influences on the size of refined austenite grain.

The High Nb Content Microalloying Approach for Developing High Performance Structural Steel

Two kinds of ULCB steel were designed with different Nb content. The yield strength of the steel which was made by adding 0.09%Nb, TMCP (Thermo Mechanical Controlling Processing) and tempering can achieve more than 900MPa. As compared with 0.05%Nb steel, adding 0.09%Nb can refine the austenite grain significantly. Through the analysis by TEM and EDX, a large amount of Nb precipitation particles can be observed in the steel with 0.09% Nb tempered at 650°C for 1h and these particles are generally small than 20nm.

Transformation Behavior of Precipitates in a W-alloyed 10 wt pct Cr Steel for Ultra-supercritical Power Plants

Thermodynamic calculation, thermal analysis, and identification and observation of precipitates have been carried out on a W-alloyed 10 wt pct Cr steel by means of ThermoCalc program, differential thermal analysis (DTA), X-ray diffraction (XRD) and transmission electron microscopy (TEM) with an energy dispersive X-ray spectrometer, respectively. Several critical phase transformation points were determined by combining experimental results with calculations. Two individually stable phases of Nb(C, N) and VN, instead of one single phase MX (X: C, N), M(23)C(6) or Laves phases, were predicted in the calculated equilibrium phase diagram. An unstable elongated M(7)C(3) with relatively higher Cr was detected unexpectedly in the normalized and tempered steel. Two kinds of spherical Nb(C, N) with different size were recognized as the primary and the secondary precipitates of Nb(C, N) which contain different V contents. It was observed that one kind of complex precipitate in a special V-wing shape was resulted from two plate-like VN adhering to coarse primary spherical Nb(C, N).

Effect of Si on Precipitation Behavior of Nb-Laves Phase and Amount of Nb in Solid Solution at Elevated Temperature in High Purity 17%Cr-0.5%Nb Steels

The effect of Si was investigated on the precipitation behavior and the amount of Nb in solid solution at temperature ranging from 1073 to 1173 K in high purity 17Cr-0.5Nb steels. Adding Si promoted the precipitation of the Nb Laves phase and then decreased the solubility of Nb in steels. The Nb Laves phase which was composed of Fe, Cr and Nb could be expressed as (Fe, Cr) 2 Nb in a 0.002 mass% Si steel. On the other hand, in a 0.5 mass% Si steel the Nb Laves phase which was composed of Fe, Si, Cr and Nb could be expressed as (Fe, Si, Cr) 2 Nb. Based on calculations from the experimental results assuming the Laves phase is Fe 2 Nb, the standard free energy change of the Fe 2 Nb precipitation reaction was about ―61 k J/mol for a 0.002 mass% Si steel.

Synthesis and Dielectric Properties of Niobia Coating on BaTiO3

Different from conventional powder mixing, this study demonstrates a method of homogeneous coating for niobia (Nb2O5) on ferroelectric barium titanate (BaTiO3) powders. The precipitation of Nb coating with pH has been determined quantitatively by the inductance‐coupled plasma method. Crystalline phases were determined by X‐ray diffractometry, and the formation of a “core‐shell” structure with a Nb concentration gradient was observed by a transmission electron microscope with X‐ray energy‐dispersive spectroscopy. The dielectric constant (K) of the samples prepared by the nanocoating method showed a more stable temperature coefficient of capacitance and well satisfied the requirements of X7R.

Microscopic examination of an Alloy 600/182 weld

A microscopic study was carried out to examine the microstructural and compositional features of an Alloy 600/182 weldment. The grain boundaries of the Alloy 600 base metal consisted of low angle boundaries (4.4%), coincidence site lattice boundaries including twins (46.6%), and random high angle grain boundaries (49.0%). The precipitates in the matrix of Alloy 600 were identified as Cr 7C 3, irrespective of the intergranular and intragranular ones. The microstructure of the Alloy 182 weld metal consisted of cellular dendrites in the grains epitaxially solidified from the heat affected zone, and the Mn concentration increased periodically across the dendritic interfaces. Contrary to the Alloy 600 base metal, most of the grain boundaries of the Alloy 182 weld metal were low angle and random high angle grain boundaries, with a negligible fraction of coincidence site lattice boundaries. Tiny Cr-rich M 23C 6 and Nb carbides were distributed on the grain boundaries in the weld metal, and a Cr-depletion zone was formed due to the Cr carbide precipitation. Precipitates of (Nb,Ti)C, Al 2O 3 type and TiO 2 type oxides were found in the Alloy 182 weld metal.