Isochronal phase transformations of low-carbon high strength low alloy steel upon continuous cooling

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 + pearlite, acicular ferrite (AF), and bainitic ferrite (BF) due to the cooling rates increasing from 5 to 3000 degrees C min(-1), 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.