铌-钒微合金钢中碳氮化合物的析出特点

用三维原子探针分析了Nb—V微合金钢中析出相的特征,发现不同大小的析出团簇成分有较大的差异。结果表明：碳原子首先在位错等缺陷处偏聚,然后钒和铌原子依次向碳原子偏聚处聚集,先后形成V-C和V-Nb—C原子团簇,进而发展成（Nb,V）C复合相,并随着其进一步长大,成分出现不均匀。当合金中存在氮元素时,也会形成（Nb,V）（N,C）复合相。     

Strain-Induced Precipitation Kinetics of Vanadium Carbonitride Precipitates with the Cubic Structure in High-Strength Weathering Steels

Strength of Materials - The strain-induced precipitation kinetic model of vanadium carbonitride [V(C, N)] precipitates with the cubic structure in vanadium-nitrogen (V–N) microalloyed...     

Effect of nitrogen and vanadium on austenite grain growth kinetics of a low alloy steel

Austenite grain growth kinetics in a steel containing 0.4% C, 1.8% Cr with different nitrogen contents (in the range 0.0038–0.0412%) and a micralloying addition of 0.078% V were investigated. The investigations were carried out in an austenitising temperature range of 840–1200 °C for 30 min. The results of investigations showed that N promotes the grain growth of austenite. The microalloying addition of vanadium protects the austenite grain growth because of carbonitride V(C,N) precipitation and the grain boundary pinning effect of undissolved particles of V(C,N). Using a thermodynamic model, the carbonitride V(C,N) content, undissolved at the austenitising temperature was calculated. At temperatures when a coarsening and dissolution of carbonitride occurs, the austenite grains start to growth. The effect of nitrogen on the type of chord length distribution of austenite grains was analysed.     

Vanadium high-strength low-alloy steels for low-temperature use

High-strength low-alloy (HSLA) steels having low impact transition temperature are possible substitutes for costlier 2 1/2% and 3 1/2% nickel steels. The effects of solid solution strengthening, grain size and precipitation in ferrite on the strength and toughness of low-carbon steels and the special advantages of vanadium as an alloying element in HSLA steels, are discussed. An investigation has been carried out with 1.5% manganese low-carbon steels containing vanadium in the range 0.12% to 0.29% and 0.013% to 0.017% nitrogen. Room temperature tensile and sub-zero temperature impact tests down to–100° C, and a metallographic study to determine the grain sizes and pearlite contents of the steels normalized at different temperatures, have been carried out. Calculations are made with empirical equations for yield and tensile strengths and the values obtained are compared with those experimentally observed. The solubility products of vanadium carbide and vanadium nitride are calculated and compared with available data to throw light on the mechanism of strengthening of the steels.     

Strengthening mechanisms in high-strength microalloyed steels

Abstract

The mechanisms responsible for strengthening a series of high-strength cold-rolled steels with tensile strengths up to 800 MN m^?2 have been investigated. The magnitude of precipitation strengthening in the annealed steels is shown to be in agreement with the Orowan-Ashby model for non-deforming particles. Strengthening depends only upon the volume fraction and diameter of the precipitates, and is not influenced by their chemical composition, nor by whether the precipitation-hardening elements are added singly or in combination. Manganese alone is a weak solid-solution-strengthening agent, but has a synergistic effect in combination with titanium or niobium, which is attributed to its depression of the austenite-ferrite transformation and precipitation temperatures. Vanadium is a much less efficient strengthening element than titanium or niobium in annealed steel, owing to the rapid coarsening rate of vanadium carbonitride precipitates and the considerable loss in strength on processing from hot-rolled coil to annealed sheet. Sulphur acts to reduce the strength of the annealed steels and tends to coarsen the grain structure. The physical basis of this effect is not known, but it is suggested that it may be associated with the partial solution of managanese sulphide during slab reheating and its subsequent re-precipitation during hot rolling. Phosphorus and nitrogen are the most efficient strengthening agents up to tensile strengths of 450 MN m^?2, but stronger materials require a combination of strengthening modes, depending upon the application.

MST/111     

The effect of micro alloying elements (vanadium,titanium) additions on the austenite grain growth behavior in medium carbon steel containing nitrogen

The austenite grain growth behavior of microalloying elements free steel (nitrogen steel) and micro alloyed steel (V−N steel and V−Ti−N steel) was investigated. The equilibrium dissolving behavior of precipitates was calculated by thermodynamic software and the morphology was observed by tunneling electron microscope. Moreover, grain growth kinetics was analyzed through theoretical calculation. Results show that the austenite grain size of V−N steel and V−Ti−N steel are significantly refined by the undissolved precipitates compared to nitrogen steel. Due to higher dissolving temperature of (vanadium, titanium)(carbon, nitrogen), the austenite grain of V−Ti−N steel keeps fine and increases slowly from 900 °C to 1250 °C. The difference of activation energy between V−N steel and V−Ti−N steel was supposed to come from the effect of different kinds of precipitates on the austenite grain growth. Compared to the vanadium rich (vanadium, titanium)(carbon, nitrogen), the titanium rich (vanadium, titanium)(carbon, nitrogen) is larger and more stable in view of its existence at higher temperature. The decrease of pinning forces can be attributed to the decrease of volume fraction and increase of radius of (vanadium, titanium)(carbon, nitrogen). Compared with critical grain sizes, the larger experimental grain sizes lead to grain growth from 900 °C to 1250 °C.     

Evolution of precipitates,in particular cruciform and cuboid particles,during simulated direct charging of thin slab cast vanadium microalloyed steels

Abstract

A study has been undertaken of four vanadium based steels which have been processed by a simulated direct charging route with processing parameters typical of thin slab casting, where the cast product has a thickness of 50 to 80 mm (in this study 50 mm) and is fed directly to a furnace to equalise the microstructure prior to rolling. In the direct charging process, cooling rates are faster, equalisation times shorter, and the amount of deformation introduced during rolling less than in conventional practice. Samples in this study were quenched after casting, after equalisation, after the fourth rolling pass, and after coiling, to follow the evolution of microstructure. The mechanical and toughness properties and the microstructural features might be expected to differ from equivalent steels which have undergone conventional processing. The four low carbon steels (~0.06 wt-%) which were studied contained 0.1 wt-%V (V – N), 0.1 wt-%V and 0.010 wt-%Ti (V – Ti), 0.1 wt-%V and 0.03 wt-%Nb (V – Nb), and 0.1 wt-%V, 0.03 wt-%Nb and 0.007 wt-%Ti (V – Nb – Ti). steels V – N and V – Ti contained around 0.02 wt-% N, while the other two contained about 0.01 wt-%N. The as cast steels were heated at three equalising temperatures of 1050°C, 1100°C, or 1200°C and held for 30 – 60 min before rolling. Optical microscopy and analytical electron microscopy, including parallel electron energy loss spectroscopy (PEELS), were used to characterise the precipitates. In the as cast condition, dendrites and plates were found. Cuboid particles were seen at this stage in steel V – Ti, but they appeared only in the other steels after equalisation. In addition, in the final product of all the steels, fine particles were seen, but it was only in the two titanium steels that cruciform precipitates were present. PEELS analysis showed that the dendrites, plates, cuboids, cruciforms, and fine precipitates were essentially nitrides. The two Ti steels had better toughness than the other steels but inferior lower yield stress values. This was thought to be, in part, due to the formation of cruciform precipitates in austenite, thereby removing nitrogen and the microalloying elements, which would have been expected to precipitate in ferrite as dispersion hardening particles.     

Dispersion strengthening in vanadium microalloyed steels processed by simulated thin slab casting and direct charging Part 1 – Processing parameters,mechanical properties and microstructure

Abstract

A study simulating thin slab continuous casting followed by direct charging into an equalisation furnace has been undertaken based on six low carbon (0·06 wt-%) vanadium microalloyed steels. Mechanical and impact test data showed that properties were similar or better than those obtained from similar microalloyed conventional thick cast as rolled slabs. The dispersion plus dislocation strengthening was estimated to be in the range 80–250 MPa. A detailed TEM/EELS analysis of the dispersion sized sub 15 nm particles showed that in all the steels, they were essentially nitrides with little crystalline carbon detected. In the steels V–Nb, V–Ti and V–Nb–Ti, mixed transition metal nitrides were present. Modelling of equilibrium precipitates in these steels, based on a modified version of ChemSage, predicted that only vanadium rich nitrides would precipitate in austenite but that the C/N ratio would increase through the two phase field and in ferrite. The experimental analytical data clearly point to the thin slab direct charging process, which has substantially higher cooling rates than conventional casting, nucleating non-equilibrium particles in ferrite which are close to stoichiometric nitrides. These did not coarsen during the final stages of processing, but retained their highly stable average size of, ～7 nm resulting in substantial dispersion strengthening. The results are considered in conjunction with pertinent published literature.     

Aluminium nitride precipitation in multiple microalloyed pipeline steels

Abstract

The precipitation of aluminium nitride has been studied in pipeline steels containing aluminium, niobium, nitrogen, titanium, and vanadium. It was found that the titanium to nitrogen ratio had a dominant influence on the formation of aluminium nitride as reflected in the high value of aluminium in these precipitates when the ratio was low. Thermomechanical deformation of the steels appeared to promote aluminium nitride precipitation which occurred preferentially in the coarser particles. The precipitates of multiple microalloyed steels were found to be complex and their occurrence was interpreted in terms of isomorphology and miscibility. The overall steel analyses have been utilised in charting the sequence of precipitation.

MST/793     

Formation of precipitates in multiple microalloyed pipeline steels

Abstract

An investigation has been carried out to identify the precipitates in multiple microalloyed steels. The microalloying elements and interstitials included aluminium, niobium, titanium, vanadium, carbon, and nitrogen. It was found that the precipitates are complex in nature and they were rationalised on the basis of mutual solubility probably enhanced by non-stoichiometry. The precipitate morphologies were interpreted mainly in terms of steel compositions. Steels quenched from 1250°C contained titanium rich precipitates accompanied by the evolution of new niobium rich precipitates after hot rolling and quenching. A parameter K₁ indicative of solute participation in the precipitation phenomenon was established and showed excellent correlation between steel and precipitate analyses. A sequence of precipitation in multiple microalloyed steels was achieved using solubility relationships as a premise.

MST/803     

低碳含V微合金钢析出物的双亚点阵模型热力学研究

钢中碳氮析出物通过细晶强化和析出强化方式对钢的力学性能有非常重要的作用.基于规则溶液的双亚点阵模型(其中一个为金属亚点阵,另一个为间隙原子亚点阵)建立了碳氮化钒及碳氮化钛的热力学计算模型,用以研究析出物的析出开始温度、给定温度的奥氏体成分.经计算得V(CxN1-χ)、Ti(CxN1-χ)的析出温度分别是833℃、1343℃,最大的摩尔分数为7.8×10-4.     

Influence of hydrogen on mechanical properties of nitrogen supersaturated austenitic stainless steels

Abstract

Alloying austenitic stainless steels with nitrogen up to a concentration of 1 wt-% improves yield strength, tensile strength, and ductility. Further increase in the nitrogen concentration results in chromium nitride precipitation at the grain boundaries and a decrease in the ductility with a change in the fracture mode from ductile to intergranular. Hydrogen charging causes high reversible dilatation in the lattice and remarkable reduction in the ductility. The ductility losses caused by hydrogen are more pronounced at higher nitrogen concentrations and a change of the fracture mode from intergranular to transgranular is observed in steels with more than 1 wt-% nitrogen. Chromium nitride precipitates are shown to have an insignificant role in the hydrogen embrittlement. Hydrogen charging steels with nitrogen concentrations of below 1 wt-% enhances the strengthening effect of nitrogen but, at higher nitrogen concentrations, hydrogen is shown to be detrimental to the strength.     

Review of Z phase precipitation in 9–12 wt-%Cr steels

For high temperature applications, 9–12?wt-%Cr steels in fossil fired power plants rely upon precipitate strengthening from (V,Nb)N MX nitrides for long term creep strength. During prolonged exposure at service temperature, another nitride precipitates: Cr(V,Nb)N Z phase. The Z phases lowly replace MX, eventually causing a breakdown in creep strength. The present paper reviews the Z phase and its behaviour in 9–12?wt-%Cr steels including thermodynamic modelling, crystal structure, nucleation process and precipitation rate as a function of chemical composition. The influence of Z phase precipitation upon long term creep strength is assessed from several different 9–12wt-%Cr steel grades and alloy design philosophies.     

The effect of titanium and reheating temperature on the microstructure and strength of plain-carbon,vanadium- and niobium-microalloyed steels

A series of plain-carbon, vanadium- and niobium-microalloyed steels with or without titanium addition were used to evaluate the effect of a small amount of titanium addition on the properties of steels. Titanium inhibits austenite grain coarsening during reheating and grain refinement was observed when the reheating temperature was below the austenite grain coarsening temperature. The lower the reheating temperature, the less was the observed precipitation strengthening effect of V(C, N). The addition of titanium to microalloyed steels reduces the precipitation strengthening effect of V(C, N) but has no visible effect on that of Nb(C, N). The mechanism of reducing the strengthening effect of V(C, N) is possibly caused by the depletion of available nitrogen content for V(C, N) formation.     

Microstructural evolution during thermomechanical processing of microalloyed medium carbon steels

Abstract

A laboratory study was carried out to determine the characteristics of austenite grain growth and recrystallisation, strain induced precipitation, and continuous cooling transformation kinetics for two microalloyed medium carbon steels (1541 + Ti,V and 1541 + Nb). Austenite grain refinement is achieved by a combination of undissolved carbonitride precipitates at the reheat temperature, deformation recrystallisation at temperatures above T _NR and strain induced carbonitride precipitation. Deformation below T _NR promotes transformation to grain boundary ferrite (GBF), intragranular ferrite (IGF), and pearlite (P) at the expense of bainite (B) in both steels. This is attributed to increased density of nucleation sites for ferrite and pearlite at austenite grain boundaries, twin boundaries, and deformation bands. The results suggest that thermomechanical forging schedules could be designed to produce refined F + P microstructure, and hence, to realise improved strength, toughness, and machinability in the forging.     

Microstructure and mechanical properties of medium-carbon ferrite–pearlite steel microalloyed with vanadium

Abstract

Microstructural analysis and mechanical testing have been carried out on medium-carbon steels to which additions of vanadium in the range 0·075–0·6 wt-% were made. The steels were either continuously cooled or isothermally heat treated after austenitization. Vanadium carbide precipitation in the proeutectoid ferrite regions of the microstructure and, more unusually, also in the pearlitic ferrite lamellae, were identified by transmission electron microscopy. Moreover, in both ferrite phases the precipitates are aligned in rows, indicative of interphase precipitation at the austenite/ferrite transformation interface. These observations are discussed in terms of the various mechanisms that have been proposed for the interphase precipitation reaction. In the alloys studied the vanadium additions were found to increase the strength of the steels by up to 100%, but to reduce the ductility and notched impact resistance. The most useful combination of increased strength with reasonable ductility and impact toughness was achieved with an addition of 0·15 wt-% V. The vanadium additions contributed to a number of variations in microstructure and therefore in strengthening mechanisms, but the largest effect was the interphase precipitation strengthening of the ferritic phases. The highest strength levels were achieved in fully pearlitic microstructures with the pearlitic ferrite lamellae strengthened by interphase precipitation of the vanadium carbide.

MST/536     

Vanadium microalloying for ultra-high strength steel sheet treated by hot-dip metallising

Ultra-high strength steel sheets have been subjected to heat treatments that simulate the thermal cycles in hot-dip galvanising and galvannealing processes and evaluated with respect to their resulting mechanical properties and microstructures. The steels contained suitable contents of carbon (～0.2%), manganese (1.2%) and chromium (0.4%) to ensure that they could be fully transformed to martensite after austenitisation followed by rapid cooling in a continuous annealing line, prior to galvanising. Different contents of vanadium (0–0.1%) and nitrogen (0.002–0.012%) were used to investigate the possible role of these microalloying elements on the strength of the tempered martensite. Vanadium, especially when in combination with a raised nitrogen content, helps to resist the effect of tempering so that a larger proportion of the initial strengthening is preserved after the galvanising cycle, giving tensile strength levels exceeding 1000?MPa. Different deoxidation practices using aluminium or silicon have also been included. These showed similar strength levels at corresponding carbon contents but the bendability of the Si-killed steel sheet was considerably superior. Microstructural examinations have been made on the annealed steels but the reason for the beneficial effect of vanadium is still not fully explained. It is concluded that microalloying with vanadium is a very promising approach in the development of corrosion-resistant ultra-high strength steel sheet products.     

Structure and strength of AlN/V bonding interfaces

AlN ceramics are bonded using vanadium metal foils at high temperatures in vacuum. Different bonding temperatures were used in the range 1373–1773 K with bonding times of 0.3–21.6 ks. The AlN/V interfaces of the bonded joints were investigated using SEM, electron probe microanalysis and X-ray diffraction. A bonding temperature of 1573 K was found to be suitable to activate both parts to initiate a phase reaction at the interface, because a thin V(Al) solid solution layer formed adjacent to the ceramic at 1573 K just after 0.9 ks, and a small flake-shaped V₂N reaction product formed inside the vanadium central layer. The formation of V(Al) and V₂N controls the interfacial joining of the AlN/V system at 1573 K up to 5.4 ks bonding time. The pure vanadium layer quickly changed to vanadium-containing V₂N. The diffusion path could be predicted for the AlN/V joints up to 0.9 ks at 1573 K following the sequence AlN/V(Al)/V₂N/V, while after 0.9 ks, the interface structure changed to AlN/V(Al)/V₂N + V by the growth Of V₂N into the vanadium. The AlN/V joints shovyed no ternary compounds at the interface. A maximum bond strength could be obtained for a joint bonded at 1573 K after 5.4 ks having a structure of AlN/V(Al)/V₂N + V. At 7.2 ks, nitrogen, resulting from AlN decomposition, escaped and the remaining aluminium reacted with V(Al) to form V₅Al₈ intermetallic, which is attributable to the decrease in bond strength.     

Characterisation of Laves phase precipitation and its correlation to creep rupture strength of ferritic steels

The Laves phase precipitation process was characterised by means of field emission scanning electron microscopy to demonstrate its effect on creep rupture strength of steels with a fully ferritic matrix. To eliminate the effects of carbide and carbonitride precipitations so that the creep rupture data can be analysed exclusively in relation to the Laves phase precipitation process, an alloy Fe–9Cr–3Co–3W (wt.%) without C and N additions was used for the study. Creep rupture strengths were measured and volume fraction and particle size of Laves phase precipitates in the ruptured specimens were analysed. It was found that the creep rupture strength started to collapse (or decrease more rapidly) long before the Laves phase precipitation reached equilibrium fraction. This was related to the onset of the coarsening of Laves phase particles, which precipitated only on grain boundaries and hence contributed little to precipitation strengthening. Creep deformation had no effect either on the precipitation kinetics or on the growth kinetics of Laves phase particles.     

Phase formation and properties of vanadium-modified Ni–Cr–B-Si–C laser-deposited coatings

A Ni–Cr–B-Si–C alloy powder was modified by addition of 2 and 5 wt% of vanadium to tackle the high cracking sensitivity of the original composition during laser deposition. The effects of vanadium on microstructure and phases were investigated by Scanning Electron Microscopy, Energy Dispersive Spectroscopy, and Transmission Electron Microscopy (TEM) and the changes in the hardness and cracking tendency of the deposits were evaluated. In comparison to the original composition, V-modified alloys produced deposits with lower hardness and moderately reduced cracking tendencies. Addition of vanadium transformed the nature and the morphology of the boride precipitates and added VC particles to the microstructure but did not induce a significant microstructural refinement. TEM characterizations confirmed that borides phases in the modified deposits consisted of alternating layers of CrB and (Cr_1?xV_x)B but the VC existed as independent particles which were formed on the boride precipitates. The final phase constitution of the modified alloys was dramatically influenced by the complete solid solubility between CrB and VB and the lack of solubility between Cr₇C₃ and VC. Addition of vanadium did not provide the phases which could act as nucleation sites to refine the microstructure of the deposits because VB had a tendency to dissolve in CrB and VC was formed at low temperatures on the boride phases. The outcomes of this study can be used to evaluate the effects of adding early transition metals such as vanadium on the microstructure and phase formations of the Ni–Cr–B-Si–C alloys.