Microstructure and properties of Ti–Nb–V–Mo-alloyed high chromium cast iron

The correlations of microstructure, hardness and fracture toughness of high chromium cast iron with the addition of alloys (titanium, vanadium, niobium and molybdenum) were investigated. The results indicated that the as-cast microstructure changed from hypereutectic, eutectic to hypoeutectic with the increase of alloy contents. Mo dissolved in austenite and increased the hardness by solid solution strengthening. TiC and NbC mainly existed in austenite and impeded the austenite dendrite development. V existed in multicomponent systems in forms of V alloy compounds (VCrFe₈ and VCr₂C₂). With the increase of alloy additions, carbides size changed gradually from refinement to coarseness, hardness and impact toughness were increased and then decreased. Compared with the fracture toughness (6 J/cm²) and hardness (50·8HRC) without any alloy addition, the toughness and hardness at 0·60 V–0·60Ti–0·60Nb–0·35Mo (wt%) additions were improved and achieved to 11 J/cm² and 58·9HRC, respectively. The synergistic roles of Ti, Nb, V and Mo influenced the solidification behaviour of alloy. The refinement of microstructure and improvement of carbides morphologies, size and distribution improved the impact toughness.     

Transformation and precipitation behaviour of Ti–Mo bearing high strength medium-carbon steel

Abstract

The authors describe here the transformation and precipitation behaviour of Ti–Mo bearing high-strength medium-carbon steel during continuous cooling, using a combination of thermo-simulation and microscopy approach. The study demonstrates that Ti and Mo carbides precipitate during austenite-to-ferrite, austenite-to-bainite and even during austenite-to-martensite transformations, contributing to precipitation strengthening. Four different types of precipitates in the size range of 3–200 nm were observed during the transformation. They are spherical (Ti,Mo)C and TiC, cuboidal (Ti,Mo)(C,N) and long thin strips of Fe_xC. The size of the precipitates was large and the density was less during austenite transformation. However, the size decreased and density increased during the austenite-to-bainitic ferrite transformation. During the austenite-to-martensite transformation, a high density of fine and spherical-shaped precipitates comprising of Ti and Mo in the range of 3–10 nm were observed.     

Incomplete transformation of upper bainite in Nb bearing low carbon steels

Abstract

Kinetics and microstructure of bainite transformation in Fe–(0·15 or 0·05)C–0·2Si–1·5Mn (mass%) alloys with Nb addition of 0·03 mass%. Bainite transformation occurs at temperatures below 873 K. At 853 K, transformation rapidly proceeds by formation of bainitic ferrite without carbide precipitation, but transformation stasis appears for a certain period in the Nb added alloys leaving untransformed austenite film between neighbouring bainitic ferrites. On the other band, the Nb free alloys do not show such a stasis until the transformation is completed. By further holding, the transformation in the Nb added alloy restarts by forming the mixture of dislocation free ferrite with cementite precipitation in the austenite films. In contrast, bainite transformation accompanying cementite precipitation occurs in both Nb free and Nb added alloys at 773 K, resulting in no difference in transformation kinetics. It is proposed that the incomplete transformation is caused by suppression of ferrite nucleation at interphase boundaries between pre-existing bainitic ferrite and austenite due to Nb segregation.     

Influence of V and Ti on hot ductility of Nb containing steels of peritectic C contents

Abstract

The hot ductility of in situ melted tensile specimens of Ti–Nb containing steels having C contents in the peritectic C range 0·12–0·17% with and without V has been examined over the temperature range 700–1000°C. An improved testing regime for simulating the continuous casting process was used, which takes into account both primary and secondary cooling conditions. For the Nb containing steels, the ductility improved in the temperature range 750–850°C as the Ti/N ratio increased. However, ductility at 800°C was still below the 35–40% reduction in area values required to avoid transverse cracking. This was attributed to the copious precipitation of sub 40 nm NbTi(CN) precipitates along the grain boundaries and finer precipitates within the grains. Adding V to the Ti–Nb containing steels resulted in significantly improved ductility with reduction in area values at 800°C in excess of 45%. This improvement was due to a decrease in the fraction of fine particles, and in accord with this better ductility, transverse cracking of industrial slabs was avoided.     

Effects of carbon and niobium on microstructure and properties for Ti bearing steels

The objective of the study described here is to elucidate the effect of carbon and niobium on the microstructure, precipitation behaviour, and mechanical properties of 0·09C–0·11Ti (%) steel and 0·05C–0·025Nb–0·11Ti (%) steel under ultra fast cooling condition. The strengthening mechanisms are also discussed. The ferrite grains size and the size of precipitates in Ti and Nb–Ti steels were measured respectively. The mechanical properties obtained in Ti steel were similar to Nb–Ti steel with yield stress >700 MPa, elongation >20%, and good low temperature impact toughness. The study underscores that addition of higher carbon content by 0·04% under controlled rolling and ultra fast cooling conditions, we can achieve similar strength in the absence of micro-alloying element, niobium.     

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.     

Influence of microalloying additions on thickness of grain boundary carbides in ferrite–pearlite steels

Abstract

For a series of plain C and microalloyed steels at two levels of Mn, the growth of grain boundary carbides has been monitored after heating to 920°C and cooling at 40 and 150 K min^?1 through the austenite–ferrite/pearlite transformation down to room temperature. In pearlite free steels, on cooling to room temperature, all the C in solution in the ferrite is able to precipitate as carbides at the boundaries and the grain boundary carbide thickness is dependent on the number of nucleation sites for precipitation. Increasing the cooling rate increases the number of sites and reduces the carbide thickness. In ferrite–pearlite steels, the grain boundary carbides form the ‘tails’ to the pearlite colonies. The thickness of the grain boundary carbide is related to the pearlite reaction, since the temperature at which this occurs controls both the thickness of the carbide nuclei and the amount of C available for precipitating out on these tails. Increasing the cooling rate and Mn content causes a decrease in the transformation temperature and leads to finer carbides. The pearlite nose transformation temperature must be ≦600°C to produce fine (≦0·2 μm) carbides. The austenite grain size, which controls the pearlite colony size, is also very important in determining the thickness of carbides, since the finer the grain size, the greater the carbide density and,for a given amount of C available for precipitation, the finer the resulting carbides. Faster cooling or a higher Mn content refine the pearlite colony size leading to finer carbides. Compared with C–Mn–Al steels, Nb and Ti microalloying additions result in coarser carbides and higher carbide densities. The increased carbide density is due to the finer austenite grain size and the coarser carbides are due to the finer grain size raising the transformation temperature. The implications of these observations on impact behaviour are discussed.

MST/1858     

Study of cast microalloyed steels

Abstract

Improved grades of cast steels were prepared by microalloying C–Mn–Cr steels (0.15–0.22C, wt-%) with V (0.1)+Ti (0.01) or V (0.11)+Nb (0.034)+Ti (0.01) combinations. When quenched and tempered an ultimate tensile strength (UTS) of the order of 784–1078 MPa (80–110 kg mm2) and an elongation ranging from 10 to 35% were achieved. However, the YS/UTS ratio was usually of the order of 0.9, which indicates that these steels did not undergo enough plastic deformation before failure under tensile stress. Examination by TEM of the as quenched as well as the quenched and tempered samples yielded the following information. All the as quenched steels had lath martensite morphology with retained austenite entrapped in the interlath region. After water quenching from 950°C an appreciable volume fraction of undissolved precipitates remained in the steels. Retained austenite broke down during tempering at 400°C and above producing a chain of rod like carbides. During tempering partial recrystallisation of the laths into polygonal ferrite occurred. Fine precipitates were deposited on the dislocation substructure.     

Effect of dissolution and precipitation of Nb on the formation of acicular ferrite/bainite ferrite in low-carbon HSLA steels

Effect of dissolution and precipitation of Nb on the phase transformation during cooling was investigated. It is firstly recognized that either the formation of acicular ferrite or the separation of bainite ferrite could be adjusted by the preparation of the steel specimens with different amounts of solute Nb and Nb-precipitates in austenite (isothermally holding at 850 °C for different durations). An increase in isothermal duration at 850 °C would spawn more Nb(CN) precipitates, leading to a microstructural evolution from bainite ferrite to acicular ferrite/bainite ferrite dual phase, and eventually to acicular ferrite in the final microstructure. This could be explained by the solution of Nb in the austenite, due to the solute dragging effect of Nb, can decrease the A_r3 temperature and promote the formation of bainite ferrite, while the precipitation of NbC can increase the A_r3 temperature and promote the formation of acicular ferrite by increasing the nucleation sites of acicular ferrite. Thus, the properties of acicular ferrite/bainite ferrite dual phase steel can generally be improved by appropriately controlling the state of Nb (Nb(CN) as precipitates and Nb in solution) in the austenite before cooling, which provides a new approach to the modification of acicular ferrite/bainite ferrite ratio.     

Aging behaviour of cobalt free chromium containing maraging steels

Abstract

This paper reports an investigation of the aging behaviour of two Co free Cr containing maraging steels (Fe–1·0Si–11·2Cr–1·3Mo–9·1Ni–1·2Al–1·0Ti and Fe–0·8Si–17·2Cr–6·1Ni–0·4Al–0·9Ti, all at.-%), using hardness measurements, electron microscopy of replicas and thin foils, atom probe field ion microscopy (APFIM), and thermochemical calculations. Two different families of intermetallic phases (Ti₆Si₇Ni₁₆G phase and η Ni₃Ti) have been found to contribute to age hardening. The composition and morphology of these precipitates were studied in deformed and undeformed alloys after aging at 420–570°C for various times. In addition, reverted austenite has been found in the aged structure. Results obtained using APFIM are compared with equilibrium thermodynamic calculations and previous APFIM studies of conventional Cr free low Al and Si maraging steels having higher Mo contents.

MST/1558     

Effects of simulated on line accelerated cooling processing on transformation temperatures and microstructure in microalloyed steels Part 1 – Strip processing

Abstract

Simulations of industrial thermomechanical processing and on line accelerated cooling of a low carbon microalloyed strip steel were carried out using a quench deformation dilatometer. Effects of processing parameters, such as accelerated cooling rate T and accelerated cooling interrupt temperature T_I on the critical transformation temperatures and final microstructure were determined. The most important on line accelerated cooling (OLAC) processing parameter is the accelerated cooling interrupt temperature, which controls whether the transformed microstructure is predominantly ferrite or bainite. A variety of (Ti, Nb, Fe) carbide, nitride, and carbonitride precipitates are present in the OLAC processed samples. The final precipitate distribution is developed at three stages of processing, namely: dissolution and coarsening of pre-existing precipitates at the reheat temperature, precipitation in deformed austenite during the deformation schedule, and precipitation in ferrite after the interruption of accelerated cooling. Maximum precipitation strengthening occurs for T_I=700–640°C.

MST/3424     

Influence of aluminium on carbide precipitation in low carbon microalloyed steels

Abstract

Precipitation in an 0·1C–0·5Si–1·5Mn–0·15Mo–0·5Ni–0·05V–Fe(wt-%) alloy containing from 0·04 to 0·2 wt-%Al was examined in the tempered condition. After hot rolling, the steels were solution treated at 1200°C for 2 h, then quenched in water. Tempering was mainly carried out at 600 or 650°C for 1 h. The precipitates were identified and measured using transmission electron microscopy and energy dispersive X-ray analysis. It was found that coarsening of Fe₃C carbides in the prior austenite grain boundaries was associated with low Al content, while the size of Fe₃C particles in the lath boundaries was independent of Al content. It is proposed that Al segregated to the prior austenite grain boundaries during solution treatment, associated with vacancies, and decreased the rate of vacancy migration, thereby retarding the coarsening of cementite.

MST/1707     

Microalloying effects on microstructure and mechanical properties of 18Cr–2Mo ferritic stainless steel heavy plates

The microstructure, including grain size and precipitation, tensile strength and Charpy impact toughness of (Nb + V) 18Cr–2Mo ferritic stainless steel heavy plates with/without Ti were investigated by means of optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and standard tensile strength and Charpy impact toughness testing. It was found that for 18Cr–2Mo heavy plate, a good combination of Nb–V stabilized method without Ti induces refinement of grain sizes due to the precipitation of amounts of fine Nb carbonitrides and V nitrides. Meanwhile, the mechanical testing results indicate that optimal transformation of grain size, precipitation that Nb–V composition system brings to 18Cr–2Mo heavy plate is beneficial to improvement of strength and impact toughness.     

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.     

Composition and crystallography of nitride precipitates in an austenitic stainless steel containing niobium and vanadium

Abstract

The composition and crystallography of small nitride precipitates in a 17Cr–12.5Ni low-carbon austenitic stainless steel, containing 0·5 at.-% V + Nb and 0·4 at.-%N, have been studied using atom-probe field-ion microscopy and transmission electron microscopy. Precipitates resulting from static aging at 750°C for 8,100, or 1170 h were all found to contain N, V, Nb, and Cr, and small amounts of Fe and C. The stoichiometry, close to MX_0·5, remained almost constant, but the Nb content decreased and the V and Cr contents increased with longer aging times. Electron diffraction showed that the precipitates analysed had the fcc NaCI structure, but with a superlattice with a lattice parameter (a =0·838±0.004 nm) twice as large as that of the subcell. The appearance of the long-range-ordered MX_0·5 phase, its relationship to other phases, and the compositional changes occurring during aging are discussed and compared with earlier observations.

MST/170     

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     

Effects of non-recrystallization zone reduction on microstructure and precipitation behavior of a ferrite-bainite dual phase steel

A dual-phase pipeline steel with high deformability was processed by a two-stage controlled rolling and followed by relaxation process before accelerated cooling. The mechanical properties of the samples were studied, and the effects of different reductions in the second rolling stage on the microstructure and precipitation behavior of the steel were analyzed using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), positron annihilation technique (PAT) and physicochemical phase analysis. The samples mainly consisted of ferrite, bainite, and a small amount of martensite/austenite (M/A) islands. When the reduction increased to 80% in the second rolling stage, lower yield ratio and better impact toughness were obtained, and the microstructure with 49.6% (volume fraction) ferrite and higher dislocation density became more homogenous. The nominal chemical formulas of M(C, N) for sample A and B were (Nb_0.735Ti_0.260Cr_0.005)(C_0.420N_0.580) and (Nb_0.757Ti_0.237Cr_0.006)(C_0.464N_0.536), respectively and the phase structure of precipitates of both samples were almost the same. The strain induced more precipitation of M(C, N) in the non-recrystallization zone of austenite than in the intercritical zone. Also the fraction of M(C, N) and the precipitation strengthening effectiveness were promoted by the increase of reduction.     

Microstructure characterization of nanometer carbides heterogeneous precipitation in Ti–Nb and Ti–Nb–Mo steel

The influences of micro-alloying elements and hot deformation on the precipitation morphology of Ti–Nb and Ti–Nb–Mo steels were investigated. The nanometer sized carbide particles randomly dispersed in the ferrite matrix are attributed mainly to severe deformation at high temperature and low isothermal holding temperature. Of the two steels with different combinations of the micro-alloying elements, Ti–Nb and Ti–Nb–Mo, the steel with Ti–Nb–Mo was more effective in precipitating hardening due to its slower carbide coarsening rate. Based on observations of micrographs, the nano-sized TiMoC and TiNbC precipitated in polygonal ferrite grains when the Ti–Nb–Mo and Ti–Nb steels were isothermally treated at 650 °C for 3 min and 180 min. The smaller of the two carbides, TiMoC, precipitated in the ferrite grain, and the hardness of Ti–Nb–Mo steel was higher than that of Ti–Nb steel. Moreover, the tiny ferrite grains and high dislocation density in the Ti–Nb–Mo steel were found to provide an attractive combination of strength and toughness.     

Microstructure and grain boundary microanalysis of X70 pipeline steel

Grain boundaries (GBs), particularly ferrite: ferrite GBs, of X70 pipeline steel were characterized using analytical electron microscopy (AEM) in order to understand its intergranular stress corrosion cracking (IGSCC) mechanism(s). The microstructure consisted of ferrite (), carbides at ferrite GBs, some pearlite and some small precipitates inside the ferrite grains. The precipitates containing Ti, Nb, V and N were identified as complex carbo-nitrides and designated as (Ti, Nb, V)(C, N). The GB carbides occurred (1) as carbides along ferrite GBs, (2) at triple points, and (3) at triple points and extending along the three ferrite GBs. The GB carbides were Mn rich, were sometimes also Si rich, contained no micro-alloying elements (Ti, Nb, V) and also contained no N. It was not possible to measure the GB carbon concentration due to surface hydrocarbon contamination despite plasma cleaning and glove bag transfer from the plasma cleaner to the electron microscope. Furthermore, there may not be enough X-ray signal from the small amount of carbon at the GBs to enable measurement using AEM. However, the microstructure does indicate that carbon does segregate to : GBs during microstructure development. This is particularly significant in relation to the strong evidence in the literature linking the segregation of carbon at GBs to IGSCC. It was possible to measure all other elements of interest. There was no segregation at : GBs, in particular no S, P and N, and also no segregation of the micro-alloying elements, Ti, Nb and V.     

Effect of microalloying on aging of a Cu-bearing HSLA-100 (GPT) steel

Investigations were carried out on aging of a HSLA-100 steel containing Cu as the major alloying element and Nb, Ti and V as microalloying elements. The aging process after varying amounts of cold deformation was followed by hardness measurements and microstructural changes were studied using light and electron microscopy. Presence of Ti activates the formation of (Nb, Ti)C precipitates and completely suppresses the precipitation of Cu. Even a solution treatment at 1100°C is not sufficient to completely dissolve Nb and Ti in the matrix and undissolved (Nb, Ti)C precipitates were observed in oil quenched state. Strain induced aging at 400° C causes simultaneous coarsening of existing precipitates and nucleation of fresh carbides, which results in multi-stage hardening in this steel. Strong precipitate-dislocation interactions cause retardation in recrystallization of deformation structure leading to retention of high hardness levels even on prolonged aging