A new ultrahigh strength niobium-microalloyed pipeline steel of yield strength ∼700 MPa has been processed. The Charpy impact toughness at 0 °C was 27 J and tensile elongation was 16%. The ultrahigh strength is derived from the cumulative combination of fine grain size, solid solution strengthening with additional interstitial hardening, precipitation hardening from carbides, dislocation hardening, and mixed microstructure. The microstructure was characterized by polygonal ferrite, upper bainite, degenerated pearlite, and martensite–austenite (MA) constituents. The microstructure of weld and heat-affected zone (HAZ) was similar to the base metal such that the hardness is retained in the weld region implying insignificant softening in the weld zone. Niobium and titanium precipitates of different morphology and size range evolved during thermomechanical processing and include rectangular (∼500 nm), irregular (∼240–500 nm), cuboidal/spherical (∼125–300 nm), and very fine (

Microstructure and high strength–toughness combination of a new 700 MPa Nb-microalloyed pipeline steel

The paper describes the microstructural constituents in a industrially processed Nb-microalloyed pipeline steel having yield strength of ∼620 MPa. The microstructure of base, heat affected zone (HAZ), and weld metal of the fabricated steel pipe was examined by optical and transmission electron microscopy. The microstructure of thermomechanically processed pipeline steel primarily consisted of non-equiaxed ferrite of mixed morphologies with small fraction of degenerated pearlite. The microstructure contained high dislocation density, sub-boundaries and dislocation substructures. The HAZ was characterized by a combination of fine and coarse grained polygonal ferrite structure with high density of dislocations and fine cementite particles. In the weld metal, the constituents of complex ferrite were low temperature transformation products formed during continuous cooling such as quasi-polygonal or massive ferrite, acicular ferrite, bainitic ferrite and dispersion of coarse and fine cementite particles in the ferrite matrix. The precipitates in the investigated pipeline steel were of duplex type containing either Nb and Ti or Ti and Mo, even though the steel contained low concentration of titanium. Precipitates of different morphology and size range were observed and include rectangular (∼100–130 nm), cuboidal/spherical (∼20–100 nm), fine (∼10–20 nm) and very fine (

Microstructure of high strength niobium-containing pipeline steel

We describe here the precipitation behavior and mechanical properties of 560 MPa Ti–Nb and 770 MPa Ti–Nb–Mo–V steels. The precipitation characteristics were analyzed in terms of chemistry and size distribution of precipitates, with particular focus on the crystallography of precipitates through an analysis of electron diffraction patterns. In addition to pure carbides (NbC, TiC, Mo 2 C, and VC), Nb containing titanium-rich carbides were also observed. These precipitates were of a size range of 4–20 nm. The mechanism of formation of these Ti-rich niobium containing carbides is postulated to involve epitaxial nucleation of NbC on previously precipitated TiC. Interface precipitation of NbC was an interesting observation in compact strip processing which is characterized by an orientation relationship of [0 0 1] NbC //[0 0 1] α-Fe , implying that the precipitation of NbC occurred during austenite–ferrite transformation.

Fine-scale precipitation and mechanical properties of thin slab processed titanium–niobium bearing high strength steels

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.

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

Chemical composition of structural steels with a ferrite-pearlite microstructure has been developing towards decreasing carbon content, to increase weldability, with increased microalloying element content, to provide grain refinement, solid solution and precipitation strengthening. During the UOE forming of large diameter (more than 400 mm) welded pipes the strength drop from plate to pipe, as a result of reverse deformation (the Bauschinger effect), depends on steel grade, namely microalloying element content, and processing. In this project the microstructure of two Nb- and V-microalloyed steels has been studied with optical, scanning and transmission electron microscopy. The dislocation density and (Ti,Nb,V,Cu)-rich particle diameter, volume fraction and number density were measured for as-rolled and annealed (30 min. at 400 \(^0\)C and 550 \(^0\)C) steels. The Bauschinger effect was measured during compression-tension testing for the same steel conditions. The yield stress drop during reverse deformation has been found to increase with an increase in forward pre-strain, dislocation density and particle number density within the effective particle diameter range of 12-50 nm. On the basis of dislocation-particle interaction analysis, a quantitative model of work-hardening behaviour dependence on particle number density and dislocation density has been derived for the reverse deformation of studied steel grades.

https://ro.uow.edu.au/cgi/viewcontent.cgi?article=3948&context=eispapers

Bauschinger effect in Nb and V alloyed line-pipe steels

One of the requirements to successfully produce high-strength pipeline steel is the optimization of precipitation strengthening. Some high-strength pipeline grades rely on increased levels of niobium (Nb); in these grades, it is important to ensure that all of the Nb is effectively employed. It is generally accepted that the choice of coiling temperature (i.e., the temperature of steel strip just before coiling) is critical in maximizing Nb carbonitrides in ferrite. Additional control of this precipitation may be attained by deformation at these coiling temperatures, an approach termed “cool deformation.” In this work, steel specimens were heated to a temperature of 1200 °C and held for 20 minutes to ensure significant dissolution of the Nb precipitates. Some specimens were aged at 400 °C for times ranging from 10 minutes to 10 hours followed by air cooling. Others were subjected to deformation at 400 °C (cool deformation) prior to aging. It was found that cool deformation improves the mechanical properties (both yield and tensile strengths are significantly higher than those of the aged only specimens). A field emission gun—scanning electron microscope (FE-SEM) was applied successfully in the characterization of Nb precipitates using carbon extraction replicas. The FE-SEM observation of the replicas confirmed the presence of Nb precipitates in ferrite. The FE-SEM could analyze small particles (below 50 nm) embedded in the steel, but the analysis had to be carried out at low voltages to maximize spatial resolution, resulting in a poor signal. However, it is possible to analyze carbon extraction replicas in the FE-SEM using high voltages, because the interaction volume effect is no longer a problem.

Effect of cool deformation on mechanical properties of a high-strength pipeline steel

Field emission scanning electron microscopy (FESEM) was used to analyse small particles (<50 nm) in ferrite. Analysis of bulk specimens must be carried out at low voltages to preserve volume resolution, resulting in very weak signals. However, thin foils and carbon extraction replicas could be analysed with FESEM using relatively high voltages, because the interaction volume effect is no longer a problem.

Characterisation of precipitation of niobium carbide using carbon extraction replicas and thin foils by FESEM

Texture Evolution in Grain Growth of Nanocrystalline Ni

Precipitation strengthening is an important parameter controlling the mechanical properties of low carbon steels. These precipitates are very fine and are normally analyzed using either thin foils or carbon extraction replicas under a transmission electron microscopy (TEM). In this work, field emission gun scanning electron microscope (FE-SEM) was applied successfully in the characterization of niobium (Nb) carbo-nitride (C,N) precipitates using carbon extraction replicas. FE-SEM observation of high strength linepipe steel replicas before and after aging at 400°C for 1 hr confirmed the presence of Nb(C,N) precipitates in ferrite. The FE-SEM could analyze small particles (below 50 nm) embedded in the steel but the analysis had to be carried out at low voltages to maximize spatial resolution resulting in a poor signal. However, carbon extraction replicas in the FE-SEM can be analyzed using high voltages, since the interaction volume effect is no longer a problem.

FE-SEM Study of Fine Nb Precipitates in Carbon Extraction Replicas

Extended abstract of a paper presented at Microscopy and Microanalysis 2005 in Honolulu, Hawaii, USA, July 31--August 4, 2005

Microanalysis of Very Fine Precipitates by FE-SEM

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