The Impact of Isothermal Treatment on the Microstructural Evolution and the Precipitation Behavior in High Strength Linepipe Steel

Abstract

Isothermal treatment affects the microstructural evolution and the precipitation behavior of high-strength low alloy (HSLA) steels. In this regard, thermal simulation of different isothermal treatment temperatures was adopted by using a thermomechanical simulator. The results showed that hardness reached the maximum value at 600 °C holding temperature, which was related to a finer grain structure and granular bainite. The strengthening effect of precipitates was remarkable due to the combination of small particle size and small interparticle spacing. It is presumed that the precipitation started after 600 s at 600 °C. Precipitation strengthening continued to exist, even though coarsening of ferrite grains led to softening phenomena when the specimen was isothermally held at 750 °C, which led to relatively high hardness. The precipitates were fcc (Ti, Nb) (N, C) particles, and belonged to MX-type precipitates. Average size of precipitates increased from 3.14 to 4.83 nm when the specimens were isothermally held between 600 °C and 800 °C. Interparticle spacing of precipitates also increased with increasing isothermal treatment temperatures. These led to a reduction in precipitation strengthening. At the same time the polygonal ferrite content increased and ferrite grain size got larger, such that the hardness decreased continuously.

Influence of microalloying elements (Ti, Nb) and nitrogen concentrations on precipitation of pipeline steels—A thermodynamic approach

“…Nitride precipitates which form at higher temperatures are larger in size than carbonitride precipitates. The thermal simulation of coiling process for a X90 pipeline steel 1 between 873 and 1073 K demonstrated interparticle spacing increment and growth of (Ti, Nb)CN precipitates from average size of 3.14 nm for treatment at 873 K to 4.83 nm for 1073 K. The mechanical properties of the steel were affected significantly due to the reduced precipitation strengthening effect, however, these precipitates still hindered grain growth to some extent.…”

“…Depending on the chemical composition of the steel, above certain temperatures during heat treatment, rolling and/or welding, the austenite grains tend to coarsen because the precipitates are taken into solution, leaving the grain boundaries unimpeded 10‐12 . Formation and coarsening of nitride and carbonitride precipitates, particularly in microalloyed steels, have been extensively studied experimentally due to their influence on the mechanical properties of products, 1,4,8,13 the formation of cracks during shaping of steels 14‐16 and corrosion resistance due to hydrogen induced cracking 2,13 and microstructural texture 2,17,18 . However, due to the growing demand for improved mechanical properties, environmental resistance, weldability and reduced production cost of High Strength Low Alloy (HSLA) steels, new complex chemical compositions and thermomechanical processing have been continuously developed to advance high‐grade pipeline steels for practical applications 3,19‐22 .…”

“…Segregation of titanium and nitrogen at the late stages of solidification consumes a substantial fraction of the added titanium to form coarse TiN precipitates; the remaining titanium, precipitates in austenite 23 . Fine titanium nitride precipitates extend the temperature range over which the grain boundary pinning is effective, due to a significantly higher solution temperature of these precipitates compared to other precipitates that are known to restrict grain growth 1,2 . Therefore, extensive efforts have been made to optimize the Ti/N ratio as a major microstructural control factor for a variety of steels 4,5,24‐26 .…”

“…Microalloyed high strength steels are widely used as structural materials, specifically for transporting crude oil and natural gas over long distances because of their exceptional mechanical properties, corrosion resistance, and weldability compared to carbon steels 1‐3 . Nitrides, 4,5 carbides, 6,7 and carbonitrides 5,8 of microalloying elements, such as niobium, titanium, and vanadium, precipitate in austenite during thermomechanical processes, pin grain boundaries and consequently improve the mechanical properties of the steels through grain refinement and precipitation hardening 9 …”