Anisotropy and microstructural evolutions of X70 pipeline steel during tensile deformation
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ARTICLE Anisotropy and microstructural evolutions of X70 pipeline steel during tensile deformation Qiuzhi Gaoa) School of Resources and Materials, Northeastern University at Qinhuangdao, Qinhuangdao, Hebei 066004, China; School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, Wollongong, NSW 2522, Australia
Cheng Lu, Huijun Li,b) Jintao Li, Jian Han, and Liang Chen School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, Wollongong, NSW 2522, Australia (Received 26 February 2018; accepted 25 May 2018)
Tensile properties of different directions of X70 pipeline steel plate were tested, and microstructural evolutions of different zones along the transverse direction (TD) were also investigated using electron backscatter diffraction. The highest strength values (yield strength and ultimate strength) appear at TD, and the diagonal direction shows the largest uniform elongation. The elongations of the polygonal ferrite and quasi polygonal ferrite grains increase with the decrease in the distance to the fracture zone. The ratio between high-angle grain boundaries and low-angle grain boundaries in the as-received steel is about 7/3 and starts to decrease from the fillet zone to the fracture zone. The refinement of grains occurs adjacent to the fracture section with the formation of subgrains. With the increase in tensile strain, the intensities of cube and c-fiber textures increase sharply, and the reinforcement of the (111)½112 component was obviously larger than the (111)½121 component in the c-fiber texture during tensile deformation.
I. INTRODUCTION
Currently, the increasing demand for transportation of oil and natural gas over long distances calls for high performance pipeline steels which feature not only high strength but also good low-temperature toughness.1–3 Upgrading of pipeline steels from X60 to X100 and even higher grades has been achieved, aiming at the increment of transportation capacity and reduction of cost.4 With yield strength higher than 480 MPa, X70 pipeline steel has already been widely used in numerous fields with high pressure, large diameter, and long distance transmission pipelines. During fabrication of X70 pipeline steels, the thermomechanical control process (TMCP) technology has enabled to obtain high strength and good lowtemperature toughness. It is well known that anisotropy plays a significant role in microstructural evolution and mechanical properties, and much attention has been focused on control of anisotropy to improve qualities of products.5–7 Various morphologies of ferrite microstructures can be assumed in TMCP pipeline steels: polygonal ferrite (PF), quasi polygonal ferrite (QF), granular bainitic ferrite (GF), and bainitic ferrite (BF).1,8,9 PF, Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2018.190 J. Mater. Res., 2018
assuming equiaxed grains with smooth continuous boundaries, forms at the highest temperature and slowest cooling rate; QF with
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