Study of Pipeline Steels with Acicular Ferrite Microstructure and Ferrite-bainite Dual-phase Microstructure

Materials Research. 2015; 18(1): 36-41 DOI: http://dx.doi.org/10.1590/1516-1439.256813 © 2015 Study of Pipeline Steels with Acicular Ferrite Microstructure and Ferrite-bainite Dual-phase Microstructure Xiurong Zuoa*, Zhengyue Zhoua School of Physics and Engineering, Zhengzhou University, Zhengzhou, 450052, PR, China a Received: December 11, 2013; Revised: January 11, 2015 Three kinds of X70 steels with the same chemical composition and different microstructures fabricated by varying processes were compared in aspect of the microstructures and mechanical properties using SEM, TEM and tensile testing machine. The experimental results showed that the steel 1 with acicular ferritic microstructure fabricated by thermal-mechanical processing with online accelerated cooling (TMCP) exhibited an excellent combination of strength and toughness, but provided high yield ratio of 0.85, low uniform elongation of 8.3% as well as low strain hardening exponent of 0.09, indicating poor deformability. In contrast to above steel 1, the steel 2 and steel 3 having ferritebainite dual-phase microstructure respectively fabricated by TMCP and intercritical annealing exhibited the improved deformability in terms of the low yield ratio of 0.69 and 0.68, high uniform elongation of 12.8% and 11.8%, and strain hardening exponent of 0.157 and 0.155. It is argued that the optimum properties combination of strength, toughness and deformability can best be achieved by obtaining a ferrite-bainite dual-phase microstructure. This kind of ferrite-bainite dual-phase pipeline steel is appropriate to transmitting oil and natural gas in seismic zone and permafrost. Keywords: pipeline steel, dual-phase microstructure, ferrite, bainite, deformability 1. Introduction Pipeline steels are usually used to transport crude oil or natural gas over a long distance. In this condition, high strength and toughness are usually required. For X70 pipeline steel, an acicular ferritic microstructure is an optimal microstructure because of its excellent combination of strength and toughness. The microstructure of acicular ferrite consists of massive ferrite, granular ferrite, and bainitic ferrite with high density of dislocations and second phases islands in the matrix1. A low-carbon Mn-Mo-Nb-Ti microalloyed pipeline steel can promote the acicular ferrite transformation2. however, pipeline steel with this kind of microstructure possesses high yield ratio3, indicating poor deformability. Dual-phase steel (DP) with ferrite plus martensite/ bainite phases can be fabricated by heat treating in the intercritical region (α+γ) or by TMCP processes, which has a low yield ratio and rapid strain hardening rate at the onset of plastic deformation, thereby, excellent deformability can be attained4-6. Generally, the microstructure and mechanical properties of DP steel depend on their chemical composition and process parameters7-9. DP steel has been widely used in automotive industry at present. At present, long-distance oil/gas transmission with pipeline steel is the most economical and safe method, but the pipeline traversing seismic zone and permafrost is prone to deformation10. The combination of high strength, high toughness and deformability is important requirements for the pipeline steel in such a severe environment. So DP steel with an excellent combination of deformability and *e-mail: strength is a judicious selection for pipeline steel with high deformability. Acicular ferrite X70 pipeline steel and dual-phase X70 pipeline steel can be produced by two different TMCP process through the controlling of rolling parameters and cooling conditions. At the same time, dual-phase X70 steel can also been fabricated by intercritical annealing with acicular ferrite X70 pipeline steel. These three kinds of X70 steels with the same chemical composition fabricated by different processes were compared in aspect of the microstructures and mechanical properties in the present work. The results will be beneficial to the practical application of ferrite-bainite dual-phase pipeline steels with high deformability. 2. Experimental Procedures The explored material was X70 pipeline steel with a chemical composition (wt.%): 0.07C, 1.54Mn, 0.011P, 0.0009S, 0.14Si, 0.17Ni, 0.020Als, 0.06Nb, 0.17Mo, 0.013Ti and Fe balance. Two different hot rolling schedules were respectively conducted through controlling the rolling process and the cooling rate on a steckel mill in order to obtain an acicular ferrite microstructure steel named steel 1 and a ferrite-bainite dual-phase microstructure steel named steel 2. And then, the steel 1 was heat treated in the intercritical region (α+γ) obtaining a ferrite-bainite dual‑phase microstructure steel named steel 3. Microstructure examination of the steels was performed using JSM6700F scanning electron microscope. Samples were prepared following standard metallographic 2015; 18(1) Study of Pipeline Steels with Acicular Ferrite Microstructure and Ferrite-bainite Dual-phase Microstructure procedures. The polished specimens were etched with 4 pct nital solution for microstructure observation. Substructures were examined by transmission electron microscopy (TEM, JEM 2100 with 200 kV). For TEM observation, thin foils were prepared by a twin-jet polishing technique. The tensile samples with a gauge length of 50.8 mm and a gauge diameter of 38.1 mm were prepared in the longitudinal direction of the rolled plates. The mechanical properties were tested at room temperature and the tensile speed was 1.0 mm/min. In order to evaluate the low-temperature toughness, standard charpy test specimens (10×10×55 mm) were prepared and tested at 253 K (–20 °C) on an impact testing machine in the transversal direction. Drop-weight tear test (DWTT) was tested at 258 K (–15 °C) in the transversal direction. 37 3. Results and Discussion 3.1. Microstructure Figure 1 shows the microstructures of steels by different processes. The steel 1 in Figure 1a,b exhibited the acicular ferrite (AF) microstructure mainly consisting of massive ferrite, granular ferrite and bainitic ferrite with widely dispersed martensite/austenite (M/A) islands and little bainite in the matrix. The prior austenite grain boundary was eliminated, because the nucleation of AF was mainly on dislocations, inclusions such as complex oxides or sulphides and the growing austenite/ferrite interface11. By adjusting the parameters of TMCP, a dual-phase microstructure of polygonal ferrite (PF) plus bainite colonies at ferrite grain boundaries can be obtained in steel 2 (Figure 1c,d). During hot rolling process on a steckel Figure 1. SEM images of experimental steels (a)(b) Steel 1; (c)(d) Steel 2; (e)(f) Steel 3. 38 Zuo & Zhou mill, the steel was first preheated in a soaking furnace, and then cooled to the rolling temperature above nonrecrystallization to roll. And then, the steel was cooled below non-recrystallization temperature and subsequently rolled in non-recrystalliza (...truncated)