Study on the continuous cooling transformation behavior of heavy thickness X80 pipeline steel

Abstract

In this paper, the continuous cooling transformation behavior of heavy thickness X80 pipeline steel after two steps deformation was studied by Gleeble1500 thermal simulator. The continuous cooling transformation (CCT) curve of the sample was set up according to the thermal dilation curve and microstructure. The results show that during the continuous cooling process, ferrite and degenerated pearlite in the X80 are obtained in the high temperature phase transformation region. The bainite in the X80 was obtained in the medium temperature transformation region, and when the cooling rate is less than 20°C/s, the micro structure is granular bainite. When the cooling rate is between 20∼30C/s, the mixed microstructure of granular bainite and lath bainite were obtained. According to the CCT curve, in the production, the start cooling temperature and cooling rate should be controlled properly to get expected microstructure so as to ensure the property requirements of the steel plate.

Laminação Termo-Mecânica Visando a Obtenção De Um Aço Api 5l Grau X80 No Laminador De Chapas Grossas Da Gerdau - Efeitos Das Estratégias De Laminação

“…Comparing micrographs presented in Figure 2.5, the granular bainite blocks are slightly smaller in sample 1. In addition, it was also verified that there wasn't formation of degenerate pearlite or superior bainite, which is very positive under the toughness aspect [14]. However, the presence of polygonal ferrite was confirmed, present in a greater fraction in sample 4.…”

“…In strategy B, where finishing passes were applied at lower temperatures, the achieved rates were 15°C/s and 13°C/s for samples 3 and 4, respectively. With cooling rates between 13°C/s and 15°C/s, it may be possible for significant granular bainite formation under all conditions [14]. However, when considering the effect of accumulated strain in austenite and dislocation density on austenite decomposition kinetics, polygonal and quasi-polygonal ferrite could be formed [15].…”

“…However, sample 4 presents a fraction of polygonal ferrite significantly higher than that verified in sample 1, it can be assumed that the CCT diagrams of the 4 sample were shifted to the left in relation to those of the 1 sample [15]. As sample 4 was cooled at an average rate of 13°C/s, there were thermodynamic and kinetic conditions for the formation of polygonal ferrite, which was subjected to the lowest cooling rate [14]. Furthermore, this result may indicate that the deformation accumulated at low temperatures in sample 4 has contributed to the bainite transformation taking place at higher temperatures and, despite the availability of sites for nucleation, has given rise to a structure with granular bainite blocks slightly coarser than that verified for sample 1 [19].…”