カナダ鉱山冶金学会 (Canadian Institute of Mining and Metallurgy) の憲章と定款

The isothermal precipitation kinetics of carbides in both strain-free and strained austenite (γ) of a microalloyed steel were quantitatively investigated through the electrical resistivity and transmission electron microscopy. The (Nb,Ti)C carbides at the interfaces of the undissolved (Ti,Nb)(C,N) carbonitrides were observed at all temperatures in strain-free γ. However, for strain-induced precipitation, above 950 °C, the precipitation of (Nb,Ti)C carbides near the undissolved (Ti,Nb)(C,N) carbonitrides was predominant due to the recrystallization of strained γ. Meanwhile, the fine (Nb,Ti,V)C carbides were homogeneously precipitated in non-recrystallized γ at 850 °C and 900 °C, as well as near the undissolved (Ti,Nb)(C,N) particles. The electrical resistivity method was successfully used to quantitatively measure the isothermal precipitation kinetics of carbides in both strain-free and strained γ. The precipitation-time–temperature diagrams of the carbide in strain-free and strained γ, with nose temperatures of 950 °C, were generated and the precipitation kinetics were greatly accelerated by the applied strain.

Carbide precipitation kinetics in austenite of a Nb–Ti–V microalloyed 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

A series of anisothermal hot torsion tests were carried out to simulate hot rolling on a high-strength low-carbon CMnNbMoTi microalloyed steel corresponding to an industrial X80 grade for pipeline construction. Mean Flow Stress was graphically represented against the inverse of temperature to characterize the evolution of austenite microstructure during rolling, which was also studied by optical microscopy and SEM on samples quenched from several temperatures. On the other hand, particles precipitated at different temperatures during rolling were analyzed by means of TEM using the carbon extraction replica technique and their size distribution and mean size were determined, as well as their morphology, nature and chemical composition. The effect of rolling temperature and austenite strengthening obtained at the end of thermomechanical processing on final microstructure and precipitation state was studied. Austenite strengthening was characterized by means of the parameter known as accumulated stress (Δ σ ). It was found that ferrite grains are finer and more equiaxed when the austenite is more severely deformed during finishing (higher values of Δ σ ) but lower values of Δ σ generate a higher density of acicular structures after cooling, which should improve the balance of mechanical properties. The increase in strength associated to acicular ferrite compared to polygonal ferrite is revealed by the higher values of Vickers microhardness measured on samples corresponding to low Δ σ . On the other hand, (Ti, Nb)-rich carbonitrides can be found from reheating and their size keeps a constant value near 20–30 nm during thermomechanical processing. A second population of much finer (Nb, Mo)-rich carbonitrides whose size is close to 5 nm forms from lower temperatures, near 1000 °C. The accomplishment of two different levels of Δ σ at the end of hot rolling schedule does not seem to introduce major differences in precipitation state before final cooling.

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Evolution of microstructure and precipitation state during thermomechanical processing of a X80 microalloyed steel

Microalloyed steels are used widely in oil and gas pipelines They are a class of high-strength, low-carbon steels that contain small additions (in amounts less than 01 wt pct) of Nb, Ti, and/or V The steels may contain other alloying elements, such as Mo, in amounts exceeding 01 wt pct Precipitation in these steels can be controlled through thermomechanical-controlled processing, leading to precipitates with sizes that range from several microns to a few nanometers Microalloyed steels have good strength, good toughness, and excellent weldability, which are attributed in part to the presence of the nanosized carbide and carbonitride precipitates Because of their fine sizes, wide particle size distribution, and low volume fractions, conventional microscopic methods are not satisfactory for quantifying these precipitates Matrix dissolution is a promising alternative to extract the precipitates for quantification Relatively large volumes of material can be analyzed so that statistically significant quantities of precipitates of different sizes are collected In this article, the microstructure features of a series of microalloyed steels (X70, X80, and X100) as well as a Grade 100 steel are characterized using optical microscopy (OM) and scanning electron microscopy (SEM) A chemical dissolution technique is used to extract the precipitates from the steels Transmission electron microscopy (TEM) and X-ray diffraction (XRD) are combined to analyze the chemical composition of these precipitates Rietveld refinement of the XRD patterns is used to quantify fully the relative amounts of these precipitates The size distribution of the nanosized precipitates is quantified using dark-field imaging (DF) in the TEM The effects of microalloying content, finish rolling temperature (FRT), and coiling temperature (CT)/interrupted cooling temperature (ICT) on the grain size and the amount of nanoprecipitation are discussed Individual strengthening contributions from grain size effects, solid-solution strengthening, and precipitation strengthening are quantified to understand fully the strengthening mechanisms for these steels

https://sites.ualberta.ca/~hhenein/Met%20Trans%20A%2043A%20(2012)%20pp%203043-3061.pdf

S. Akhlaghi

Papers

Precipitation Behaviour of a Grade 100 Structural Steel