Dual-phase Pipeline Steel Research Articles

Effect of distribution characters of ferrite/bainite on deformation compatibility in dual-phase pipeline steel: Experimental and numerical study

The morphology distribution of microstructure has an important influence on the match of the strength and plasticity of dual-phase pipeline steel. Bainite and PF dual-phase steels with layered and network morphology were obtained by combining the Thermomechanical Control Process (TMCP) and inter-critical isothermal heat treatment. A novel experimental-numerical methodology studied the effect of phase distribution characters on the deformability of PF and bainite dual-phase pipeline steel. The plastic damage features and microscopic strain and stress distribution in both network and layered microstructure were clarified. The obvious differences in the microstructure evolution for both samples were captured by a quasi-situ tensile test. The PF phase in the network morphology sample was believed the main deformation phase, but the deformation of the PF phase in the layered morphology sample was restrained. Two three-dimensional (3D) crystal plasticity finite element models (CPFEMs) were established based on the electron backscatter diffraction (EBSD) data. The simulation results indicated that the strain field in layered microstructure was more uniform than that in network microstructure, and the continuous internal stress relaxation (ISR) in the bainite phase led to plastic damage at low tensile strain level; The strain field in network microstructure mainly existed in the PF phase, the deformation of the PF phase can be fully utilized. When changing the tensile x-direction of the layered microstructure to the z-direction, the strain field showed an obvious anisotropy, and loading in the z-direction increased the risk of interlaminar crack formation. • Quasi-situ tensile test was used to study the deformation behavior • Strain/stress distribution characteristics were clarified using CPFEM method • Uniform strain distribution and ISR in layered microstructure decreased its plasticity • z-axis loading increased the risk of interlaminar crack for layered microstructure

Tensile deformation behavior of ferrite-bainite dual-phase pipeline steel

In-situ tensile test accompanied by electron backscatter diffraction (EBSD) analyses were performed on polygonal ferrite (PF) and bainite dual-phase steel, selected regions of interest were analyzed following plastic deformation of the steel. Deformation-induced crystal orientation evolution, localized strain concentration, slip transfer, and geometrically necessary dislocation (GND) density were tracked. Results revealed that heterogeneity deformation facilitated formation subregions with crystal orientation deviation in grain and fragmented the grain by the new low angle grain boundaries (LAGBs) or medium angle grain boundaries (MAGBs). The PF grains with ND//<111> preferred crystal orientation exhibited high orientation stability, and almost all load axes of the selected PF grains moved to the [101] pole, resulting in enhancing {111} <110> orientation component at high strain levels. With the lattice rotation during deformation, the high angle grain boundaries (HAGBs) can change to MAGBs, which was beneficial to maintain coordination deformation among grains. Localized strain concentration can be decreased by the slip transfer across the PF grain boundaries or bainite/PF phase boundaries, which reduced the risk of micro-void formation. Additionally, the variation of α12 GND tensor average value (Ave. α12) revealed that the ferrite was continuous plastic deformation, while the bainite occurred stage hardening. The required strain for the coordination deformation was controlled by strain hardening behavior.

Enhancing Strain Capacity by the Introduction of Pearlite in Bainite and Polygonal Ferrite Dual-Phase Pipeline Steel.

In this study the strain capacity and work-hardening behavior of bainite (B), bainite + polygonal ferrite (B + PF), and bainite + polygonal ferrite + pearlite (B + PF + P) microstructures are compared. The work hardening exponent (n), instantaneous work hardening value (ni), and differential Crussard-Jaoul (DC-J) analysis were used to analyze the deformation behavior. The best comprehensive mechanical properties were obtained by the introduction of the pearlite phase in B + PF dualphase with the tensile strength of 586 MPa and total elongation of 31.0%. The additional pearlite phase adjusted the strain distribution, which increased the initial work hardening exponent and then maintained the entire plastic deformation at a high level, thus delayed necking. The introduction of pearlite reduced the risk of micro-void initiation combined with the high frequency of high angle grain boundaries (HAGBs) in triple-phase steel, which led to a low crack propagation rate.

Investigation on micromechanism of ferrite hardening after pre-straining with different strain rates of dual-phase steel

In this work, the effects of strain rates (10−4 s−1~10−1 s−1) on ferrite hardening of dual-phase steel were investigated through pre-straining tests. The strain localization was analyzed by the evolution of Kernel average misorientation (KAM). The micromechanism of ferrite hardening was elaborated by the evolution analysis of geometrically necessary dislocations (GNDs), low angle grain boundaries (LAGB) and nano-indentation. The results showed that GNDs and LAGBs in ferrite increased with increasing the strain rate, the average KAM values and nanohardness increased accordingly. Strain localization was observed in the ferrite grains at lower strain rates (10−4 s−1~10−3 s−1), and the considerable strain localization occurred at ferrite/bainite (F/B) interface at higher strain rates. Due to the nearly saturated GNDs and LAGBs density at the strain rate of 10−2 s−1, the completely ferrite hardening occurred and its nanohardness was close to dual-phase interface. During the tensile deformation after pre-straining, the yield ratio increased while the uniform elongation decreased with strain rates of pre-straining. It was worth noting that the completely ferrite hardening led to the early plastic deformation of tensile tests was interface deformation. This work provides a theoretical basis for the ferrite hardening behavior after pre-straining at different strain rates, and then contributes to the safe service of dual-phase pipeline steel.

The Microstructure Evolution of Dual-Phase Pipeline Steel with Plastic Deformation at Different Strain Rates

Tensile properties of the high-deformability dual-phase ferrite-bainite X70 pipeline steel have been investigated at room temperature under the strain rates of 2.5 × 10−5, 1.25 × 10−4, 2.5 × 10−3, and 1.25 × 10−2 s−1. The microstructures at different amount of plastic deformation were examined by using scanning and transmission electron microscopy. Generally, the ductility of typical body-centered cubic steels is reduced when its stain rate increases. However, we observed a different ductility dependence on strain rates in the dual-phase X70 pipeline steel. The uniform elongation (UEL%) and elongation to fracture (EL%) at the strain rate of 2.5 × 10−3 s−1 increase about 54 and 74%, respectively, compared to those at 2.5 × 10−5 s−1. The UEL% and EL% reach to their maximum at the strain rate of 2.5 × 10−3 s−1. This phenomenon was explained by the observed grain structures and dislocation configurations. Whether or not the ductility can be enhanced with increasing strain rates depends on the competition between the homogenization of plastic deformation among the microconstituents (ultra-fine ferrite grains, relatively coarse ferrite grains as well as bainite) and the progress of cracks formed as a consequence of localized inconsistent plastic deformation.

Effects of non-recrystallization zone reduction on microstructure and precipitation behavior of a ferrite-bainite dual phase steel

A dual-phase pipeline steel with high deformability was processed by a two-stage controlled rolling and followed by relaxation process before accelerated cooling. The mechanical properties of the samples were studied, and the effects of different reductions in the second rolling stage on the microstructure and precipitation behavior of the steel were analyzed using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), positron annihilation technique (PAT) and physicochemical phase analysis. The samples mainly consisted of ferrite, bainite, and a small amount of martensite/austenite (M/A) islands. When the reduction increased to 80% in the second rolling stage, lower yield ratio and better impact toughness were obtained, and the microstructure with 49.6% (volume fraction) ferrite and higher dislocation density became more homogenous. The nominal chemical formulas of M(C, N) for sample A and B were (Nb0.735Ti0.260Cr0.005)(C0.420N0.580) and (Nb0.757Ti0.237Cr0.006)(C0.464N0.536), respectively and the phase structure of precipitates of both samples were almost the same. The strain induced more precipitation of M(C, N) in the non-recrystallization zone of austenite than in the intercritical zone. Also the fraction of M(C, N) and the precipitation strengthening effectiveness were promoted by the increase of reduction.

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

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 ferrite-bainite 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.

Influence of Dual-Phase Microstructures on the Properties of High Strength Grade Line Pipes

The influence of dual-phase microstructures on mechanical properties of X70, X80, and X90 line pipes is investigated. It is found that the line pipes with dual-phase microstructures possess both larger uniform elongation and higher hardening exponent, especially for high grade steel X90. The tensile deformation of dual-phase line pipe does not follow the trend predicted by the Hollomon formula, and a stable strain-hardening exponent is not found. This stress-strain behavior is different from the normal line pipe. In the initial stage of plastic deformation, the strain-hardening capacity of dual-phase line pipe increases rapidly. However, it reaches a stable stage after 2.0% total strain. The dual-phase pipeline steel is composed of soft phase (polygonal ferrite) and hard phase (bainite), and thus the relatively soft ferrite is good for its deformability. Besides, the fraction of large angle grain boundaries in the dual-phase microstructures is greater than that of the normal line pipe, which is proven to be critical for improving the resistance to plastic deformation and crack propagation.

Ferrite Evolution During Isothermal Process in a High Deformability Pipeline Steel

In order to ensure the safety of long-distance oil and natural gas transmission pipeline installed in seismic and/or permafrost region, high strength pipeline steel with excellent deformability has been developed. The ferrite and bainite dual phase pipeline steel is a very important kind of high deformability pipeline steel. Polygonal ferrite is a key microstructure in ferrite and bainite dual phase deformability pipeline steel. Ferrite evolution during isothermal process at 700 °C after 50% deformation at 800 °C was conducted by using a Gleeble-3800 thermal simulator, and microstructure was characterized by using an optical microscope, a scanning electron microscope and a transmission electron microscope. There are two types of ferrite, ferrite with high density dislocation and ferrite with a little dislocation. There is about 7% (volume percent) deformation induced ferrite (DIF) for compression of 50% at 800 °C and strain rate of 1 s−1. During the isothermal process at 700 °C, with the holding time increasing, ferrite volume percent, ferrite grain number and average ferrite grain size increase. As the holding time is prolonged, dislocation recovery occurs in DIF. There are secondary phases in ferrite when the holding time is too long, and secondary phases and dislocation formation in dislocation pinning.

Effect of volume fraction of bainite on microstructure and mechanical properties of X80 pipeline steel with excellent deformability

Pipeline steel with excellent deformability and dual-phase microstructure consisting of ferrite and bainite was obtained by inter-critical accelerated cooling. Dual-phase microstructure was adopted to obtain high deformation capacity, enabling the prevention of structural failure during geological disasters such as landslides and earthquakes. The effect of the volume fraction of bainite on the mechanical properties of the dual-phase pipeline steel was investigated by microscopic analysis and testing of mechanical properties. Results indicate that both the yield strength and ultimate tensile strength of the steel increases almost linearly with the increasing volume fraction of bainite, whilst ductility, work hardening exponent, and impact absorption energy decrease. At a 50% volume fraction of bainite, the yield strength, yield strength/tensile strength ratio (Y/T), work hardening exponent, uniform elongation, and impact absorption energy of X80 dual-phase pipeline steel are 665 MPa, 0.8, 0.12, 8%, and 245 J, respectively. At this stage, the microstructure of the dual-phase pipeline steel consists mainly of small bainitic ferrite, supplemented by small polygonal ferrite with high-density dislocation. This dual-phase microstructure endows the pipeline steel with excellent strength and toughness, as well as good deformation characteristics.

Microstructure and properties of pipeline steel with a ferrite/martensite dual-phase microstructure

Abstract In order to satisfy the transportation of the crude oil and gas in severe environmental conditions, a ferrite/martensite dual-phase pipeline steel has been developed. After a forming process and double submerged arc welding, the microstructure of the base metal, heat affected zone and weld metal was characterized using scanning electron microscopy and transmission electron microscopy. The pipe showed good deformability and an excellent combination of high strength and toughness, which is suitable for a pipeline subjected to the progressive and abrupt ground movement. The base metal having a ferrite/martensite dual-phase microstructure exhibited excellent mechanical properties in terms of uniform elongation of 7.5%, yield ratio of 0.78, strain hardening exponent of 0.145, an impact energy of 286 J at − 10 °C and a shear area of 98% at 0 °C in the drop weight tear test. The tensile strength and impact energy of the weld metal didn't significantly reduce, because of the intragranularly nucleated acicular ferrites microstructure, leading to high strength and toughness in weld metal. The heat affected zone contained complete quenching zone and incomplete quenching zone, which exhibited excellent low temperature toughness of 239 J at − 10 °C.