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

Strengthening Mechanisms and Their Relative Contributions to the Yield Strength of Microalloyed Steels

Quantification of dislocation structures from anelastic deformation behaviour

Low-carbon micro-alloyed steel containing coarse MnS inclusions was subjected to different thermo-mechanical processing routes to evaluate the effect of microstructure on its Charpy impact properties over a range of temperatures. MnS inclusions were found to deteriorate the upper shelf energy (USE) of the steel but its effect on the impact transition temperature was not as detrimental as that due to the presence of TiN particles. MnS inclusions were responsible for the initiation of micro-voids; however, the propagation of the cracks from these depends on the effective grain size and the strength of the matrix. An increase in the density of dislocations and low-angle boundaries enhances the strength and the strain-hardening ability of the ferrite matrix. This helps in retarding the growth of micro-voids, thereby reducing USE and promoting cleavage crack propagation resulting in an increase in its impact transition temperature. Refinement of effective grain size, on the other hand, increases the crack propagation resistance and therefore, improves the low-temperature toughness of the steel. Finish rolling of the steel just above the Ar3 temperature (austenite to ferrite transformation start temperature) or a simple normalizing treatment of the as-rolled plates at a low austenitization temperature is recommended from the point of view of higher impact toughness and lower impact transition temperature.

Effect of microstructural parameters, microtexture and matrix strain on the Charpy impact properties of low carbon HSLA steel containing MnS inclusions

An ultra-low carbon Ti-containing microalloyed steel has been thermo-mechanically controlled rolled and water-quenched. Microstructural characterization of the samples finish rolled in the temperature range of 750–850 °C revealed the presence of polygonal ferrite, quasi-polygonal ferrite, acicular ferrite and granular bainite. Micro-texture study showed the dominance of random texture and cube orientation at higher deformation temperatures, whilst, α-fiber and γ-fiber components intensified with the decrease in finish rolling temperature. Characteristic ferrite-bainite microstructure with fine ferrite grain size (

Microstructure, texture, property relationship in thermo-mechanically processed ultra-low carbon microalloyed steel for pipeline application

One of the options available for coping with current property requirements is to increase microalloying contents. As well as affecting austenite microstructural evolution, this can lead to a larger contribution of precipitation and dislocation density hardening to the mechanical strength. However, it also results in more complex interactions between the strengthening mechanisms, making it difficult to optimize operation parameters. In this work, plane strain compression tests were carried out to produce different austenite microstructures with two low carbon steels microalloyed with different Nb levels (0.04% and 0.11%), followed by coiling simulations in the 500 to 700 °C temperature range. The mechanical properties of the transformed microstructures were determined via tensile testing and the specimens were characterized using optical microscopy, EBSD and TEM. Similar austenite grain sizes and strain accumulation levels were obtained for both steels, and as a result, it was observed that increasing Nb content had a small effect on the grain size of the transformed microstructures. On the other hand, the coiling temperature significantly affected the mechanical strength. The lowest strength values are obtained for 700 °C; these increase significantly at 600 °C (≅90 and 140 MPa increase in the Yield Strength (YS) for the low and high Nb steels, respectively) and decrease slightly at 500 °C. At the conditions investigated, similar dislocation density levels were determined for both steels, which indicates that the larger YS increase observed for the high Nb steel must be mainly due to the effect of precipitates formed during or after phase transformation.

Effect of thermomechanical treatment and coiling temperature on the strengthening mechanisms of low carbon steels microalloyed with Nb

Microalloyed steels possess good strength and toughness, as well as excellent weldability; these attributes are necessary for oil and gas pipelines in northern climates. These properties are attributed in part to the presence of nanosized carbide and carbonitride precipitates. To understand the strengthening mechanisms and to optimize the strengthening effects, it is necessary to quantify the size distribution, volume fraction, and chemical speciation of these precipitates. However, characterization techniques suitable for quantifying fine precipitates are limited because of their fine sizes, wide particle size distributions, and low volume fractions. In this article, two matrix dissolution techniques have been developed to extract precipitates from a Grade100 (yield strength of 690 MPa) microalloyed steel. Relatively large volumes of material can be analyzed, and statistically significant quantities of precipitates of different sizes are collected. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) are combined to analyze the chemical speciation of these precipitates. Rietveld refinement of XRD patterns is used to quantify fully the relative amounts of the precipitates. The size distribution of the nanosized precipitates is quantified using dark-field imaging in the TEM.

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Matrix Dissolution Techniques Applied to Extract and Quantify Precipitates from a Microalloyed Steel

The effort that the materials science and metallurgical engineering communities have expended in developing high-fidelity process models can be exploited using model predictive control (MPC) technologies to provide very high process performance. This article presents a survey of the development of high-quality models and a discussion of how these could be used to improve process operation. A summary of the current state-of-the-art in process automation technologies and the common framework in which these technologies are deployed is presented, along with a tutorial on MPC within a materials-processing context.

J. Barry Wiskel

Papers

Strengthening Mechanisms and Their Relative Contributions to the Yield Strength of Microalloyed Steels

Matrix Dissolution Techniques Applied to Extract and Quantify Precipitates from a Microalloyed Steel

Exploiting model fidelity to control metals processing