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Aimed to acquire optimum comprehensive properties for the oil and gas pipeline steels, thermal treatment should be controlled to achieve ideal microstructures. Effects of cooling rates on transformation kinetics and microstructures of the low-carbon high strength low alloy (HSLA) steel were investigated to obtain an optimized thermal treatment technology. Dilatometric measurements, light microscopy, scanning electron microscopy, and transmission electron microscopy were employed in present work. The transformed microstructures contained polygonal ferrite þ pearlite, acicular ferrite (AF), and bainitic ferrite (BF) due to the cooling rates increasing from 5 to 30008C min À1 , in present investigated HSLA steel. The result shows that, the increase of cooling rate accelerates AF transformation and refines the steel's matrix. The morphology of martensite/austenite structures transformed from islands in AF to films in BF with the increase of cooling rate.

Tempered martensitic structure is the service condition of T91 ferritic steel after adopting the austenitizing followed by tempering. Needle‐like M3C particles are precipitated during air cooling after austenization, while the precipitation of M3C is suppressed during the water cooling. The effect of existence of M3C on the precipitation behaviors of M23C6 during the early stage of tempering, as nucleation site, number density and size distribution, was investigated by means of TEM observation. The TEM results indicate that, upon the same tempering time, the size of M23C6 is smaller and its number density is higher in the sample pre‐existing M3C than in the sample without M3C. This can be explained that existence of M3C results in more M23C6 precipitates forming inside of grain, where a relatively low self‐diffusion coefficient of alloy element leads to M23C6 hardly coarsening. However, with the prolongation of tempering time, this effect becomes weaken. Microhardness results indicate that the existence of M3C phase results in the increase of hardness after tempering due to the precipitation of finer and denser M23C6 particles.