Effect of Microstructure on the Yield Ratio and Low Temperature Toughness of Linepipe Steels.

Abstract

Linepipe steels, which transport oil and gas, should have high strength, high toughness, excellent corrosion resistance and superior weldability. [1][2][3] The current demand is towards the larger-diameter and higher-pressure linepipes to improve the efficiency of transmission, resulting in more stringent specifications for the linepipe steels. In addition to the above mentioned properties, the steels should have low yield ratio (yield strength/tensile strength) for the safety concern. The lower yield ratio means the higher resistance to deformation from yielding to plastic instability, 4,5) preventing the sudden decrease in the strength. According to the engineering specifications, API X-70 steels should have the yield strength higher than 480 MPa, impact energy larger than 100 J at Ϫ40°C and the yield ratio smaller than 85 %. It has been shown in the previous study that the achievement of low yield ratio (around 80 %) and high toughness (ductile-brittle transition temperature of around Ϫ100°C) is possible in the 440 MPa grade C-Mn steel by controlling the thermomechanical process.6) However, it becomes more difficult to maintain adequate values of yield ratio and toughness as strength increases to a higher level. The basic difficulty in optimizing these properties comes from the fact they are often inversely correlated, i.e. an increase in the strength is achieved at the expense of the yield ratio and toughness, and vice versa. Therefore to develop the high performance linepipe steels, the individual effect of microstructural features on these properties should be clearly understood.One of the ways of decreasing the yield ratio is the utilization of hard second phase in the microstructure. Numerous studies on so-called dual phase steels showed that these steels have low yield ratio due to the presence of hard martensite or bainite in soft ferrite matrix. 7,8) Shikanai et al.,9) analyzed the relationship between the volume fraction and morphology of second phase and the yield ratio by finite element method (FEM) and reported that the steel should have soft matrix with around 50 % volume fraction of hard second phase to obtain low yield ratio. It has also been suggested that the larger difference in the strength between the two phases is more desirable to obtain low yield ratio.9,10) However these dual phase steels generally have low yield strengths which do not meet the property requirement for X-70 linepipe steels. Moreover the large difference in the strength between the two phases might have a deleterious effect on toughness since cracks can easily nucleate at the hard second phase particles at low temperatures. Recently, more emphasis is placed on developing acicular ferrite or bainite base steels for linepipe applications.11-13) However, the toughness of these steels is often disappointing. Therefore new microstructures are needed to obtain the adequate combinations of strength, toughness and yield ratio. ISIJ International, Vol. 42 (2002) The present study aims at elucidating the effects of microstructu...

Effect of Finish Rolling Temperature on the Microstructure and Tensile Properties of Nb–Ti Microalloyed X90 Pipeline Steel

“…With the further application of pipeline steels in long-distance transportation and harsh environment, excellent toughness, weldability and deformability become more and more important, in addition to the high strength, to guarantee the safety [2][3][4]. Appropriate design of the microstructure is essential to achieve these targeted mechanical properties, which can be obtained by optimizing the chemical composition and the Thermo-Mechanical Control Processing (TMCP) schedule.…”

Effect of the Cr Content and Coiling Temperature on the Properties of Hot Rolled High Strength Lower Bainitic Steel

“…Furthermore, there is a tendency at 500°C to have a higher fraction of secondary phases (martensite, austenite, degenerated pearlite (i.e. pearlite with an imperfect lamellar structure)), which will lead to a more continuous yielding behavior and a higher strain hardening, 18,19) which is also apparent in the lower yield ratio R p0.2 /R m . The austenite observed in most of the materials could in principle contribute to the strain hardening as well through Transformation Induced Plasticity (TRIP), but the volume fractions are quite low.…”

The Role of the Bainitic Packet in Control of Impact Toughness in a Simulated CGHAZ of X90 Pipeline Steel

“…For lath bainite or martensite, the austenite grain is divided into packets (bundled by group of blocks with the same habit plane), and each packet is further subdivided into blocks (bundled by laths with the same orientation) [12][13][14]. Early works by Lee et al [15] and Kim et al [16] indicated that prior austenite is the effective microstructure unit in control of the toughness of bainite steels, and crack can propagate across the packet boundaries without changing the direction. On the contrary, studies by Wang et al [14] and Rancel et al [17] showed that packet boundaries can strongly hinder fracture propagation and can act as an effective microstructure unit in control of the toughness for cleavage fracture.…”

“…The studies by Wang et al [14] and Rancel et al [16] indicated that the packet boundaries can strongly hinder fracture propagation and can act as an effective microstructure unit for cleavage fracture. Moreover, in recent work by Yang et al [18,34], they revealed that high angle boundary (including packet boundary) remarkably contributes to a good toughness of the CGHAZ in high-grade pipeline steel.…”