Microstructure Of X70 Pipeline Steel Research Articles

Evaluation of Mechanical Properties and Microstructure of X70 Pipeline Steel with Strain-Based Design

The microstructure and mechanical properties of X70 pipeline steel with a ferrite/martensite dual-phase microstructure produced by thermo-mechanical controlled processing were investigated by tensile tests, Charpy V-notched (CVN) impact tests, drop-weight tear tests, guided-bend tests, scanning electron microscopy and transmission electron microscopy combined with thermodynamic simulation analysis. All the mechanical properties met the strength, ductility, toughness and deformability properties requirements of X70 grade pipeline steel with strain-based design. The shear fracture area and absorbed energy of CVN at −10 °C were >97% and >205 J in base metal (BM), weld metal (WM) and heat affected zone (HAZ) with low transition temperature, indicating adequate resistance to propagating fracture. The microstructure of WM was mainly intragranular acicular ferrite that can guarantee high strength, toughness and over matching requirements of the welded joint. Because of being exposed to successive heat inputs, the ferrite plus martensite/bainite microstructure of BM was heated between Ac1 and Ts forming the HAZ. However, a high CVN impact toughness of 345 J at −10 °C in HAZ was obtained, which indicated that the excellent mechanical properties of BM would not be seriously deteriorated during the welding thermal cycles with the reasonable addition of Ti and Nb.

The Influence of Centerline Segregation on the Mechanical Performance and Microstructure of X70 Pipeline Steel

Centerline segregation is an inherent phenomenon during solidification. It is controlled by optimizing the continuous casting process, but is not easy to be avoided. According to the severity of centerline segregation of the slab, it affects the mechanical properties and final microstructure of rolled strip. The objective of this paper is to study the influence of centerline segregation degree on the microstructure and mechanical properties of X70 pipeline steels. In this study, the mechanical properties of strips produced from the slabs are examined and the results show that centerline segregation is some adverse effect on Charpy impact toughness and tensile properties of hot‐rolled strips. The strips from slab with higher degree of segregation, exhibits lower Charpy impact toughness, higher tensile strength, higher microhardness. The segregated degree of slab is studied by using the method of Mannesmann, indicating that it could be up to class 3–4. Moreover, the results from Electron Probe Micro Analyzer (EPMA) shows that high level of Mn content is obtained at the centerline of strips processed from the slab with segregation degree of class 3–4. The corresponding microstructure is identified as lath martensite, which should be contributed to the undesirable mechanical properties.

Microstructure and Mechanical Properties of X70 Pipeline Steel with High H<sub>2</sub>S Resistance

Anti-H2S X70 pipeline steel was developed. The microstructure of X70 pipeline steel was studied by the analysis of OM, SEM and TEM. The precipitation behavior was discussed. The comprehensive mechanical properties, HIC and SCC performance were systematically studied. The results indicated that the microstructure of the experimental steel was mainly acicular ferrite and granular bainite. The second phase precipitates dispersedly distributed in the matrix. The experimental steel possessed excellent strength, plasticity, low temperature toughness and low yield ratio. And therefore, the X70 pipeline steel in the study is suitable for sour service with the high strength, excellent toughness and low HIC&SSC susceptibility.