Nb Precipitation Research Articles

Effects of sulphur on hot ductility of niobium containing low carbon steels during low strain rate deformation

The effects of sulphur on hot ductility of niobium steels, in which cracking susceptibility on the continuously cast slab surface is highest, have been studied by means of hot tensile testing with particular emphasis on the segregation of sulphur atoms to the matrix/grain boundary Nb(C,N) precipitate interfaces. When low manganese niobium steels are solution treated at high temperature and then deformed at temperatures ranging from the low temperature γ to the γ/α duplex phase regions, two troughs appear in the ductility versus strain rate curve, accompanied by intergranular fracture of γ, at strain rates of ~1 s−1 and 10−3−10−4 s−1. The loss of ductility at high and low strain rates is caused by dynamic precipitation of iron rich (Fe,Mn)S and Nb(C,N) particles, respectively, both within γ grains and on the γ grain boundaries. The dependence on sulphur content is obvious at high strain rates, but it is found that the loss of ductility owing to Nb(C,N) precipitation is also reduced by decreasing the sulphur content to less than 10 ppm. This can be explained by the reduced segregation of sulphur atoms to the grain boundary Nb(C,N) precipitate/matrix interfaces, leading to suppressed decohesion and consequent nucleation of microvoids which result in ductile intergranular fracture of γ.MST/1425

Recrystallization behaviour of a CMn steel, a titanium-interstitial-free steel and a niobium-microalloyed steel during strip rolling

In order to characterize the Lake Erie strip mill of Stelco, Canada, three grades of steel (CMn, interstitial free (IF) and niobium microalloyed) were subjected to torsional simulation. The flow stresses and microstructural behaviours have been compared. As anticipated, for long interpass times (typical of plate-rolling schedules) the temperature T nr of no recrystallization can be clearly established only for the niobium steel, the other grades undergoing nearly full recrystallization at all temperatures. Short interpass times, typical of strip-rolling schedules, lead to little pass-to-pass strain accumulation in the three steels. In the temperature range 1000-900 °C, static recrystallization is largely responsible for the high degree of interpass softening in the CMn steel. In the case of the IF steel, during the latter stages of finishing, there is some strain accumulation, and both static and dynamic recrystallization occur to a significant degree. For the niobium steel, because of solute effects, the short interpass times (about 1 s) do not permit much static recrystallization. Instead, softening is brought about mostly by dynamic and post-dynamic recrystallization. This is possible because there is considerably less Nb(C, N) precipitation during the short interpass times than under reversing (plate) mill conditions. The decreased level of precipitation in the niobium steel followed by the initiation of dynamic recrystallization would lead to lower rolling loads in the strip mill for this grade than in the plate mill at the same temperature.

Influence of roughing rolling passes on kinetics of strain induced precipitation of Nb(C,N)

Influence of roughing rolling passes on kinetics of strain induced precipitation of Nb(C,N)

Influence of roughing rolling passes on kinetics of strain induced precipitation of Nb(C,N)

AbstractThe effects of reheating temperature and of the strain and temperature of roughing deformation passes on the kinetics of strain induced precipitation after a finishing deformation pass have been investigated for several niobium high strength low alloy (HSLA) steels. Strain induced precipitation was detected via its strengthening effect on a second finishing deformation carried out either by experimental rolling or by plane strain compression tests. Precipitates were also observed using thin foil electron microscopy. Decreasing reheating/roughing temperature and increasing roughing strain were found to significantly accelerate precipitation – this acceleration is attributed to clustering of niobium in solution.MST/1409

Effect of Ni3Nb precipitation on the corrosion resistance of Inconel alloy 625

Alloy 625 is susceptible to precipitation of Ni 3 Nb upon exposure to temperatures in the range of 650 to 870°C. Since this phase is low in chromium content as shown later, it can be expected to degrade the chemical stability of the alloy in oxidizing media. It is the objective of this investigation to determine the effect of Ni 3 Nb precipitation in alloy 625 on its corrosion resistance to an oxidizing medium (nitric acid)

Modeling of flow stress and rolling load of a hot strip mill by torsion testing.

In order to characterize the Lake Erie Works hot strip mill, Stelco Steel, two grades of steel (C-Mn and Nb) were subject to torsional simulation. The flow stresses and rolling loads are considered in this report. The microstructural behaviours of the two grades are compared in a separate publication. As anticipated, for long interpass times, the Tnr temperature (temperature of no-recrystallization) can be clearly established only for the Nb steel, the other grade under-going nearly full softening, i.e. recrystallization, at all temperatures. By contrast, for short interpass times of the order of 2 s, typical of the strip mill, there is little pass-to-pass strain accumulation in either steel. In the temperature range 1 020 to 920°C, static recrystallization is largely responsible for the hight degree of interpass softening in the C-Mn steel. In the case of the Nb steel, because of solute effects, the short interpass times do not permit much conventional recystallization. Instead, softening is brought about mostly by dynamic and post-dynamic recrystallization. This is possible because there is considerably less Nb(CN) precipitation during the short interpass times than under reversing (plate) mill conditions. The decreased level of precipitation in the Nb steel, followed by the initiation of dynamic recrystallization, leads to lower rolling loads in the strip mill for this grade than in the plate mill at the same rolling temperatures.

Ｘ線回折法による６０キロ級熱延高強度薄鋼板の低サイクル疲労損傷

Low cycle fatigue properties of two types of formable 60kgf/mm2 class hot-rolled high strength sheet steels for automobile use were studied by the X-ray diffraction and TEM methods. Si-Mn dual phase high strength steel sheets, with duplex microstructures of ferrite and martensite, showed initial cyclic hardening followed by cyclic softening on the stress response curves in the strain-controlled fatigue tests. Nb bearing precipitation hardened steel sheets, however, showed monotonuous cyclic softening.These cyclic properties are explained as follows:(1) The initial increase followed by a decrease in the X-ray half value breadth with increased cycles observed on Si-Mn dual phase steel results from fine cell structures in the ferrite matrix of the soft phase.(2) The monotonuous decrease in the X-ray half value breadth observed on Nb precipitation hardened steel results from subboundaries due to the rearrangement of the dislocations.

Combined APFIM–TEM study of Nb(CN) precipitation in HSLA steel

The strengthening of high strength low alloy (HSLA) steel has been attributed to the strain induced formation of small carbonitride precipitates during processing that inhibit recrystallisation of the austenite and produce a fine grained steel. The size and number density of the small Nb(CN) precipitates that formed in a HSLA steel containing 0·05 wt-% Nb have been measured after various stages of the thermomechanical process by atom probe field ion microscopy (APFIM) and transmission electron microscopy (TEM). The complementary results indicated that recrystallisation of the austenite during thermomechanical processing was inhibited by the precipitates, and that the mechanism was consistent with a subgrain boundary pinning model.MST/657

Combined APFIM–TEM study of Nb(CN) precipitation in HSLA steel

Combined APFIM–TEM study of Nb(CN) precipitation in HSLA steel

Effect of precipitation on hot ductility of niobium and aluminium microalloyed steels

The precipitation reactions occurring in C–Mn–Al and C–Mn–Nb steels before and after hot deformation have been examined and their influence upon hot ductility is discussed. Precipitation has been studied at 850°C, when ductility is poor, and also at 1100°C, when the ductility is good. Rapid intergranular precipitation occurred at 1100°C, but the precipitation present before deformation did not prove to be detrimental to ductility and grain boundary mobility at this temperature. Although only a limited amount of precipitation occurred at 850°C before deformation, intergranular precipitation continued during deformation resulting in embrittlement of the steels. At this temperature, strain induced transgranular precipitation of Nb(CN) occurred in the C–Mn–Nb steel and this is thought to be a major cause of poor hot ductility in this steel. By holding the steels for 15 min at 800–850°C before reheating to 1100°C, the area fraction of intergranular precipitation at 1100°C was increased. This produced a decrease in ductility at this temperature in the C–Mn–Al steel but had a less marked effect in the C–Mn–Nb steel.MST/107

Effect of composition and process variables on Nb(C, N) precipitation in niobium microalloyed austenite

Nucleation theory and the solubility product of niobium, carbon, and nitrogen in austenite have been used to derive equations for the start of Nb (C, N) precipitation as a function of temperature and composition. The predicted curves have been compared with the experimental observations of several authors to determine the effects of thermomechanical processing variables on the density of preferred nucleation sites and to incorporate these in the equations. Good agreement between the predicted and observed forms of precipitation curve is obtained with consistent constants in the equations when account is taken of the influence of different methods of detecting the onset of precipitation. Combining the calculated precipitation start curves with the dependence of recrystallization kinetics on composition and thermomechanical process variables when all niobium is in solution leads to prediction of the lower temperature limit for complete recrystallization and of the upper temperature limit for effective stoppage of recrystallization by precipitation. The predictions are in good agreement with observed results.MST/495

Effect of composition and process variables on Nb(C, N) precipitation in niobium microalloyed austenite

Effect of composition and process variables on Nb(C, N) precipitation in niobium microalloyed austenite

Precipitation of Nb(CN) during high strain rate compression testing of a 0.07 Pct Nb-bearing austenite

The recrystallization of austenite in plain carbon steel (C-Mn) following high temperature deformation is a rapid process which may take only fractions of a second to complete. ~-1~ It is also well known 1"2 that additions of minute amounts of elements such as niobium to a C-Mn austenite strongly delay the recrystallization of austenite, and are responsible for the development of pancaked austenite during controlled rolling. While this retarding effect of niobium has been recognized for almost 20 years,11'12 there continues to exist some controversy about the nature of the mechanism responsible for the observed retardation. Some authors 13'14'15 suggest that this retardation results from solute drag effects, while others 16-18'2~ propose that it results mainly from precipitation effects. Results of recent work 7'19 indicate that recrystallization is substantially delayed in Nb-bearing austenite decarburized to carbon levels of 0.002 pct. Since precipitation has been prevented by the decarburization treatment, the observed retardation is attributed to solute drag effects alone. However, the question remains as to which mechanism is, in fact, responsible for the observed recrystallization retardation, when precipitation is allowed as a competing process at the normal carbon levels (->0.02 pct C) typical of microalloyed (MA) steels. Retardation of recrystallization of the microalloyed austenite unequivocally attributable to precipitation effects has been extensively reported from results of experiments performed using different techniques. These include direct studies of precipitation using electron microscopy of thin foils and replica specimens, 7'16-18'2~ gross precipitate extraction, 5'~9 and indirect interrupted mechanical testing methods.6 10.13-15,17-19 However, in most of these studies, the lack of rapid quenching prevented reliable metallographic observations from being made during the first few critical seconds after the completion of the deformation. At the same time, it is well known that C-Mn austenite can undergo significant recrystallization in elapsed times of the order of one second s-1~ when deformed under conditions similar to those employed in the precipitation studies in the MA steels. The inability to observe specimens quenched within a fraction of a second has, therefore, restricted our understanding of the role played by precipitation in the early stages of retardation of recrystallization. The purpose of this paper is to demonstrate that when hot deformation is followed by rapid quenching, it is possible to show clearly that well-developed precipitates do form in austenite in less than 0.5 second. The steel investigated has the following chemical composition, in weight percent: 0,08 C, 1.25 Mn, 0.40 Si, 0.005 P, 0.005 S, 0.07 Nb, and 0.025 N. The steel was

Role of niobium in a dual-phase steel

AbstractTwo hot-rolled high-strength, low-alloy steels, one microalloyed with niobium, were intercritically annealed at 740°C for 10 min and cooled to produce a dual-phase microstructure. Although both steels showed tensile properties typical of dual-phase microstructures, only the niobium-microalloyed steel could match the mechanical characteristics of a typical commercial dual-phase steel. The role of the niobium would appear to be to provide a suitable level of strength in the initial, as-rolled state of the steel, since the properties in the dual-phase state were found to be a function of the initial state of the steel. A normalizing treatment resulted in grain refinement in both steels, but transmission electron microscopy observations showed that in the niobium-containing steel this was counterbalanced by an increase in Nb(C, N) precipitate size. The normalizing treatment thus equalized the mechanical properties of the initial state of the two steels, leading to similar dual-phase properties in both...

Effect of molybdenum, niobium, and vanadium on static recovery and recrystallization and on solute strengthening in microalloyed steels

The effect of molybdenum, niobium, and vanadium on the occurrence of static recovery and recrystallization after high temperature deformation was investigated in a series of microalloyed steels. The steels had a base composition of 0.05 pct C and 1.40 pct Mn. To this, single additions of 0.30 pct Mo, 0.035 pct Nb, and 0.115 pct V were made. Interrupted hot compression tests were performed at 900 and 1000 °C, and at a constant true strain rate of 2 s-1. The load-free time was decreased from 5000 s to 50 ms, and the degree of static softening during this period was determined. Both graphite and glass were used as lubricants. Percent softeningvs delay time curves are presented and the retarding effect of molybdenum, niobium, and vanadium addition on the rate of static recovery and recrystallization is discussed. The greatest solute retardation of static recovery and recrystallization is produced by niobium addition, followed by that of molybdenum, vanadium leading to the smallest delay. Although the rank order of this effect is the same as found under dynamic softening conditions, the relative contribution of niobium is more profound for the static condition. The solute strengthening attributable to each element was also assessed, and found to follow the same order as for the recovery and recrystallization results. At 900 °C, the onset of the static precipitation of Nb (CN) was detected at approximately 10 seconds, somewhat earlier than previously reported.

Influence du molybdene sur la recristallisation et la precipitation dynamiques dans des aciers microallies contenant du niobium et du vanadium

The influence of molybdenum on the dynamic recrystatlisation of austenite was investigated in microalloyed steels. This study was based on the measurement of the critical recrystallisation strain during compression tests operated at constant true strain rate. Additionally, the influence of molybdenum was determined on the dynamic precipitation of Nb(CN) and VN and also the hardening was calculated for additions of Nb, V and Mo.

Effect of vanadium and molybdenum addition on high temperature recovery, recrystallization and precipitation behavior of niobium-based microalloyed steels

The effects of multiple microalloy additions on the dynamic recovery and recrystallization behavior of austenite were investigated in a series of 0.05% C-l.25% Mn steels. The steels contained one or more of 0.30% Mo, 0.035% Nb or 0.115% V, and a reference plain carbon steel was also tested. Constant strain rate compression tests were conducted in the temperature range 1150°–875°C, that is, above and below the relevant carbonitride solution temperatures ( T sol ). Dynamic recrystallization-time-temperature (RTT) curves were developed from the flow curves at 5.6 × 10 −3 and 3.7 × 10 −2 s −1. At the higher strain rate, only the solute retardation of recrystallization was detected for the vanadium, Nb-V, Nb-1.9 Mn and No-Mo steels. An additional delay due to the dynamic precipitation of Nb(CN) was observed in the Nb-1.25 Mn steel, where there was a break to the right in the RTT curve. These breaks were seen in the results for all the steels at the lower strain rate. They appeared about 50°–100°C below the calculated T sol for the particular carbonitride, and coincided with the intersection of the RTT and PTT (precipitation-time-temperature) curves. The solute retarding potential of each element was normalized to 0.1 at.% with a solute retarding parameter (SRP). The SRP for niobium, molybdenum and vanadium when added singly, was 63, 10 and 3% respectively. In the multiply alloyed steels, the SRP attributable to each element was smaller than when it was present alone. Possible explanations for this negative interaction are advanced. Dynamic PTT curves for the precipitation of carbonitrides are presented for the Nb-1.25 Mn, Nb-1.9 Mn, Nb-Mo, Nb-V and Nb-Mo-V steels. The addition of molybdenum, vanadium or manganese is seen to retard the dynamic precipitation of Nb(CN). These delays are explained in terms of the decrease in T sol resulting from the reduction in the carbon and nitrogen activity coefficients in the joint presence of the former elements. An empirical formulation for the effect of manganese on NbC solubility is deduced from results reported in the literature which is consistent with the present observations.

The Effect of Hot Rolling Condition and Chemical Composition on the Onset Temperature of γ-α Transformation after Hot Rolling

The effect of hot rolling condition and chemistry of steels on the onset temperature of γ-α transformation after hot rolling was investigated using a thermal analyzer developed for the measurement of Ar3 temperature after hot rolling. The changes of Ar3 temperature and austenitic microstructure in connection with reheating temperature, rolling temperature and reduction rate were investigated in a Si-Mn and a Nb-bearing steels. The results were analyzed based on the changes of the effective interfacial area per unit volume (Sv) that included both of the recrystallized and unrecrystallized grain boundaries and deformation band as nucleation sites for ferrite. Although the Sv value increased with the refinement of recrystallized γ grain or the increase of rolling reduction below recrystallization temperature of austenite, the latter resulted in a much greater change of Ar3 temperature in the Nb steel. This was considered to be due to a reduced amount of dissolved Nb atoms around the grain boundary or deformation band through the strain induced precipitation of Nb(CN).The effect of chemistry and plate thickness on the Ar3 temperature after controlled rolling was studied in a large number of steels with different chemistry, and the relation between the Ar3 and chemistry was quantitatively established based on the multiple regression analysis.

溶接金属中での微量元素の析出とじん性の相関

This paper deals with the precipitation behavior of some microalloying elements in the weld metal during post weld heat treatment (PWHT).An investigation was carried out to define the effects of Ti, B, Nb, V etc. on notch toughness of the weld metal which was produced by the large current MIG arc process. Attention was focused into three main areas as fo flows:1. Correlation between toughness degradation attributed to PWHT and oxygen content of weld metal which governs the morphologies of Ti and B in the weld metal before PWHT.2. A quantitative estimation of increase in hardness accompanied with toughness degradation which is attributed to the precipitation of Nb as NbC.3. Determination of precipitation behavior of V existing solely or coexisting with Nb.As a result, it becomes clear that the precipitation of Ti and B as carbonitrides during PWHT is not decisive of the embrittlement of the weld metal in the oxygen range above 200 ppm, where the larger proportion of both elements exists as oxides. The precipitation hardening of Nb as carbonitrides accompanied with significant toughness degradation reaches the maximum at 600°C in the PWHT temperature range from 500°to 650°C. On the other hand, the precipitation of V is suppressed when V coexists with Nb.

Niobium carbonitride precipitation and austenite recrystallization in hot-rolled microalloyed steels

The response of austenites to thermomechanical treatments is studied in a series of niobium (columbium) HSLA steels. Interactions between composition, plastic deformation, strain-induced precipitation, and austenite recrystallization are described and related to previous work in the field. Niobium in solution prior to deformation leads to significant retardation of subsequent austenite recrystallization if Nb(C,N) precipitation takes place prior to or during the early stages of recrystallization. Such straininduced precipitation proceeds in two stages: initially at austenitic grain boundaries and deformation bands, and later on substructural features in the unrecrystallized austenite. The latter precipitation is accelerated only if it occurs in the unrecrystallized austenite; if recrystallization precedes Nb(C,N) precipitation, then the precipitation reaction is much slower. Thus, the Nb(C,N) precipitation and austenite recrystallization reactions are coupled phenomena. The conditions necessary for such an interaction are analyzed, and it is proposed that the level of supersaturation of Nb(C,N) in the austenite at the deformation temperature is a critical factor in determining whether or not an effective interaction will operate at that temperature.