Dynamic precipitation and coarsening of niobium carbonitrides during the hot compression of HSLA steels

The size distribution of the static precipitates produced in unstrained austenite was determined at 900° C by the method of extraction replication. The effect of deformation on the rate of coarsening of the particles was established up to a true strain of 0.75. Size distributions were also measured in materials containing static precipitates which were undergoing dynamic precipitation at 900°C. The mean size in these materials decreased while dynamic precipitation was occurring and then increased due to dynamic coarsening. The rate of increase duringdynamic ripening was two to three orders of magnitude higher than the rate ofstatic Ostwald ripening reported in the literature. The effect of particle size on the initiation of dynamic recrystallization was also evaluated. The results confirm the general rule that fine particles (d ≃ 6 nm) are more effective than coarser ones (e.g. d ≃ 20 nm) with respect to the retardation of dynamic recrystallization. The necessary fine particles can be nucleated homogeneously in the alpha phase or heterogeneously on gamma phase dislocations. This paper is based on a presentation made at a symposium on “Precipitation Processes in Structural Steels” held at the annual meeting of the AIME, Denver, Colorado, February 27 to 28, 1978, under the sponsorship of the Ferrous Metallurgy Committee of The Metallurgical Society of AIME.     

Precipitation behavior in ultra-low-carbon steels containing titanium and niobium

This work revealed the basic mechanism for the stabilization of carbon in ultra-low-carbon (ULC) steels that contain moderate S (0.004 to 0.010 wt pct), adequate Ti (0.060 to 0.080), and low Mn (≤0.20). During cooling through the austenitic region to the ferritic, the initially formed sulfide particles (TiS) undergo an in situ transformation into carbosulfides (H-Ti₄C₂S₂) by absorbing C and Ti. The transformation from TiS to H may be considered as a hybrid of shear and diffusion, i.e., faulted Ti₈S₉ (9R)+10[Ti]+9[C] → 4 1/2Ti₄C₂S₂ (H). At low temperature (≤930 °C), the stabilization process continues through epitaxial growth of carbides on H phase, i.e., [M]+x[C]+H → epitaxial MC _x (on H). This mechanism differs from the traditional view of stabilization, where the carbon is removed from solution by the formation of free-standing or independently nucleated H and/or MCN precipitates. While these two forms of carbon stabilization are now well known, this article presents a method of predicting which mechanism of stabilization will be operative in a given steel based on its bulk composition. Implications bearing upon new ULC steel design, considering the role of S, will be discussed.     

Carbonitride precipitate growth in titanium/niobium microalloyed steels

Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) have been used to investigate the morphology, distribution, composition, particle size distributions, and growth kinetics of carbonitride precipitates in steels containing low levels of Ti, Nb, C, and N. During the aging, only the complex carbonitride precipitates of the form (Ti_xNb_1−x) (C_yN_1-y) were found in the newly nucleated and growing particles. The youngest of these particles which approach the size of critical nuclei tends to be Ti-rich. Almost all of these particles are nearly spherical. The initial growth of the precipitates, which is very rapid, lasts less than 30 seconds followed by slow ripening. A model predicting the growth kinetics of carbonitrides and composition variation within the precipitates for the initial stage (before coarsening) has been developed based on equilibrium thermodynamics with the inclusion of capillarity and multicomponent diffusional kinetics. Satisfactory agreement with the experimental results has been demonstrated.     

铌微合金化长材产品的开发

介绍了铌微合金化技术在马鞍山钢铁股份有限公司长材产品开发中的应用情况,该公司近年来开发了铌微合金化的HRB400钢筋、HRB500钢筋、海洋石油平台用H型钢、铁道车辆用H型钢、美标MA345多级结构H型钢、耐火H型钢和MFT8非调钢盘条等长材产品,在铌微合金化长材品种钢开发方面取得了很大进展。     

Application of thermodynamic computations to the solution behaviour of niobium and vanadium carbonitrides

On the basis of well-known thermodynamic equations, a model is proposed which allows the computation of both the solution behaviour and the composition of the carbonitride precipitates of the microalloying elements in steel. Apart from the concentration of the microalloying elements and the carbon and nitrogen content, the following aspects were taken into account: the non-stoichiometric composition of carbonitrides, a regular solution behaviour of the mixture of carbide with nitride, the effect of third elements on the activity of carbon and nitrogen, the possible primary binding of nitrogen by aluminium. The calculation is based on the knowledge of the solubility products of pure carbides and nitrides. Comparison with experimental results of carbonitride precipitates in microalloyed niobium and vanadium steels support the model-based predictions concerning the temperature-dependent composition and solubility of carbonitrides under equilibrium conditions.     

Regularities of reactions of titanium carbonitrides and oxycarbides with nickel

Specific features and regularities of reactions of titanium carbide alloyed over the sublattice of nonmetals (N, O) with the nickel melt are analyzed. It is established that the partial substitution of carbon in TiC by nitrogen decreases its dissolution rate in nickel and increases the degree of process incongruence (the transfer of carbon into the melt is preferential compared with titanium). The concentration dependence of the dissolution rate of TiC_xN_z in nickel changes its sign to the opposite one compared with approaching the system to equilibrium. Titanium carbonitride is not recrystallized through the nickel solution as the only phase, and mainly its carbide component is subjected to recrystallization. It is revealed that the partial substitution of carbon in TiC to oxygen increases its dissolution rate in nickel. The dissolution of oxycarbide TiC_0.6O_0.4 in nickel is accompanied by the gradual loss of its carbon until titanium monoxide is formed and by its further disproportionation. The peculiarity of the interaction mechanism of titanium oxycarbides with the nickel melt is determined by reaction [C] + [O] = CO↑ in the liquid phase.     

Influence of microalloy type and content on induced precipitation kinetics in microalloyed steels

Using torsion tests, and applying the back extrapolation method, the statically recrystallized fraction has been determined for low carbon microalloyed steels with different V, Nb and Ti contents, at different temperatures and an equivalent strain of 0.20%. At temperatures below the static recrystallization critical temperature (SRCT), or the temperature at which strain induced precipitation commences, recrystallized fraction against time curves show a plateau caused by precipitations. The formation of the plateau makes it possible to evaluate precipitation-time-temperature (PTT) diagrams, and thus to know the precipitation kinetics. The PTT diagrams show that an increase in the microalloy content raises SRCT and reduces incubation time. For similar Nb, V and Ti contents, Nb is the element which raises SRCT the most.     

Characterisation of microalloy precipitates in the austenitic range of high strength low alloy steels

Transmission electron microscopy was used to investigate the particle size distribution, morphology, composition, and crystallography of microalloy (Ti, Nb) precipitates in 4130 steels between 900 to 1250 °C. Considering the Ti fraction, the size and the morphology, two types of precipitates were identified: cuboidal coarse TiNbMo carbonitrides, rich in TiN and a fine dispersion between 2 to 25 nm of TiNbMo carbides. The progressive formation of higher soluble phases, such as NbC and MoC, was observed on the pre‐existent, TiN and TiC precipitates. In the studied conditions, nitrides were found to be insoluble and quite resistant to coarsening. On the contrary, carbides not only began dissolution in the range of 960 ‐ 1000 °C according to the microalloy content in the steel, but also produced abnormal grain growth.     

Austenite recrystallization and carbonitride precipitation in niobium microalloyed steels

The response of austenite to thermomechanical treatment is investigated in two series of niobium microalloyed steels. Optical and electron metallographic techniques were used to follow the austenite recrystallization and carbonitride precipitation reactions in these steels. The first series of steels contained a constant level of 0.05Nb, with carbon levels varying from 0.008 to 0.25 pct. It was found that a lower carbon concentration results in faster austenite recrystallization, due to a smaller carbonitride supersaturation, which leads to a reduced precipitate nucleation rate. The second series of steels was designed with a constant carbonitride supersaturation, by simultaneously varying the Nb and C concentrations while maintaining a constant solubility product. In these steels, the recrystallization kinetics increase as the volume fraction of Nb(C, N) is reduced and/or as the precipitate coarsening rate is increased. The volume fraction of carbonitrides increases as the Nb: (C+12/14 N) ratio approaches the stoichiometric ratio of approximately 8:1. The precipitate coarsening rate was shown to increase with increasing amounts of niobium remaining in solution in the austenite (i. e., “excess” Nb after precipitation). As expected, recrystallization proceeds more slowly at lower temperatures and after a reduced amount of deformation. An experiment to determine whether Nb atoms dissolved in the austenite could exert a significant solute-drag effect on the recrystallization reaction indicated that 0.20Nb in solution could reduce the rate of recrystallization compared to a Nb-free C-Mn steel. However, this solute effect was smaller than the retarding effect which 0.01Nb can have when it is precipitated in the form of carbonitrides on the austenite substructure after rolling.     

Carbonitride precipitation in niobium/vanadium microalloyed steels

A detailed study of carbonitride precipitation in niobium/vanadium microalloyed steels is presented. A thermodynamic model is developed to predict the austenite/carbonitride equilibrium in the Fe-Nb-V-C-N system, using published solubility data and the Hillert/Staffansson model for stoichiometric phases. The model can be used to estimate equilibrium austenite and carbonitride compositions, and the amounts of each phase, as a function of steel composition and temperature. The model also provides a method to estimate the carbonitride solution temperatures for different steel compositions. Actual carbonitride precipitation behavior in austenite is then examined in two experimental 0.03Nb steels containing 0.05V and 0.20V, respectively. Samples were solution treated, rolled at 954 °C (20 pct or 50 pct), held isothermally for times up to 10,000 seconds at 843 °C, 954 °C, or 1066 °C, and brine quenched. The process of carbonitride precipitation in deformed austenite is followed by analytical electron microscopy (AEM) of carbon extraction replicas. Precipitates are. observed at prior-austenite grain boundaries, and also within the grains (presumably at substructure introduced by the rolling deformation). Analysis of the grain-boundary and matrix precipitate compositions by AEM indicates that the grain-boundary precipitates are consistently richer in vanadium than the matrix precipitates, although compositional trends with holding time and temperature are similar for the two types of precipitates. The compositions of both the grain-boundary and matrix precipitates are not significantly influenced by the rolling reduction or the holding time at temperature. As predicted by the thermodynamic model, the precipitates become more vanadium-rich as the vanadium level in the steel is increased and as the temperature is reduced. The agreement between the measured and predicted precipitate compositions is quite good for the grain-boundary precipitates, although the matrix precipitates are consistently more niobium-rich than predicted by the model.     

Carbonitride precipitation in niobium/vanadium microalloyed steels

A detailed study of carbonitride precipitation in niobium/vanadium microalloyed steels is presented. A thermodynamic model is developed to predict the austenite/carbonitride equilibrium in the Fe−Nb-V-C-N system, using published solubility data and the Hillert/Staffansson model for stoichiometric phases. The model can be used to estimate equilibrium austenite and carbonitride compositions, and the amounts of each phase, as a function of steel composition and temperature. The model also provides a method to estimate the carbonitride solution temperatures for different steel compositions. Actual carbonitride precipitation behavior in austenite is then examined in two experimental 0.03Nb steels containing 0.05V and 0.20V, respectively. Samples were solution treated, rolled at 954°C (20 pct or 50 pct), held isothermally for times up to 10,000 seconds at 843°C, 954°C, or 1066°C, and brine quenched. The process of carbonitride precipitation in deformed austenite is followed by analytical electron microscopy (AEM) of carbon extraction replicas. Precipitates are observed at prior-austenite grain boundaries, and also within the grains (presumably at substructure introduced by the rolling deformation). Analysis of the grain-boundary and matrix precipitate compositions by AEM indicates that the grain-boundary precipitates are consistently richer in vanadium than the matrix precipitates, although compositional trends with holding time and temperature are similar for the two types of precipitates. The compositions of both the grain-boundary and matrix precipitates are not significantly influenced by the rolling reduction or the holding time at temperature. As predicted by the thermodynamic model, the precipitates become more vanadium-rich as the vanadium level in the steel is increased and as the temperature is reduced. The agreement between the measured and predicted precipitate compositions is quite good for the grain-boundary precipitates, although the matrix precipitates are consistently more niobium-rich than predicted by the model.     

Interaction of titanium boride with niobium and tungsten

Summary Results are presented of investigations of the nature of the interaction of titanium boride with niobium and with tungsten in powder mixtures sintered on heating to 2850°C for TiB₂-Nb and to 2700°C for TiB₂-W.     

Interaction between deformation,recrystallization and precipitation in niobium steels

The present study was undertaken in order to establish the extent to which niobium (columbium) inhibits recovery and recrystallization of microalloyed austenite while present as solute on the one hand, and as carbonitride precipitates on the other. Three steels were used; the first was an HSLA steel containing 0.054 pct Nb, 0.05 pct C and 0.92 pct Mn. The second was prepared by treating the first in wet hydrogen at 1100°C so as to reduce the C and N content to about 10 ppm by wt. The third material was a plain carbon steel containing 0.055 pct C and 0.41 pct Mn. The isothermal recovery and re-crystallization of these materials, after an interval of hot working, was studied by means of interrupted compression tests. Samples were prestrained at 10^-2 and 10^-1s^-1 at 900, 950 and 1000°C to natural strains of 0.10 and 0.25, and held isothermally prior to reloading. The results obtained in this way, indicate that 0.56 pct of substitutional solute can give rise to an order of magnitude decrease in the rate of recrystallization. When precipitation of Nb(CN) takes place either during deformation or in strained aus-tenite the mean precipitate size is ∼ 20 nm. The presence of such particles inhibits both static recovery and recrystallization; the magnitude of the effect being dependent on the volume fraction of precipitate. When the volume fraction reaches about 0.02 pct, static recrystallization is completely suppressed for the present prestraining conditions. This paper is based on a presentation made at a symposium on “Precipitation Processes in Structural Steels” held at the annual meeting of the AIME, Denver, Colorado, February 27 to 28, 1978, under the sponsorship of the Ferrous Metallurgy Committee of The Metallurgical Society of AIME.     

ICP-AES法同时测定低合金钢中锆和铌

利用全谱ICP -AES(CID检测器 )分析技术 ,对试样溶解方法、元素分析谱线、共存元素干扰、背景校正、仪器分析参数 (射频发生器功率、雾化器压力和泵提升量 )等因素进行了研究 ,综合确定了最佳实验条件 ,并采用稀硫酸溶样后 ,经硫磷酸冒烟 ,直接进行试样前处理 ,建立了一种可同时测定低合金钢中Zr和Nb含量的简单、快速和实用的分析方法。结果表明 :本法测定钢中锆和铌含量的分析误差和精密度符合国标GB2 2 3 3 0 -94和GB2 2 3 3 9-94的技术要求 ,其检出限均为0.0 0 0     

Plastic anisotropy of low-carbon,low-manganese steels containing niobium

The effect of niobium additions (up to 0.23 pct) on the plastic anisotropy of cold-rolled and annealed low-carbon, low-manganese steels has been studied. When hot-rolling conditions involving coiling temperatures below 1150°F were used, increased concentrations of niobium were deleterious to the development of high plastic anisotropy. Isothermal transformation studies and special hot-rolling studies showed that when transformation from austenite to ferrite and precipitation of carbonitrides occurred at high temperatures (\s>1350°F), excellent plastic anisotropy (?gn about 2) was obtained after cold rolling and annealing. The results are interpreted on the basis of an effect of critical particle sizes or dispersions on the selection of preferred orientations during recrystallization. From electron microscope studies, the critical carbonitride size and interparticle spacing, necessary for the development of high plastic anisotropy, was estimated to be 40 to 500Å and 0.04 to 0.5 μm, respectively. However, the presence of a large amount of niobium in solid solution in ferrite is apparently deleterious to the development of high plastic anistropy, even though carbonitrides having the critical size and spacing are present. On the basis of these observations and other published work, it is suggested that second-phase particles influence the development of plastic anisotropy in rimmed and aluminum-killed steels as well as in niobium-and titanium-containing steels. Thus, the degree of plastic anisotropy produced in these steels is influenced by textures developed during annealing, which in turn is dependent on the dispersion characteristics of the particular second-phase particles present.