A new ultrahigh strength niobium-microalloyed pipeline steel of yield strength ∼700 MPa has been processed. The Charpy impact toughness at 0 °C was 27 J and tensile elongation was 16%. The ultrahigh strength is derived from the cumulative combination of fine grain size, solid solution strengthening with additional interstitial hardening, precipitation hardening from carbides, dislocation hardening, and mixed microstructure. The microstructure was characterized by polygonal ferrite, upper bainite, degenerated pearlite, and martensite–austenite (MA) constituents. The microstructure of weld and heat-affected zone (HAZ) was similar to the base metal such that the hardness is retained in the weld region implying insignificant softening in the weld zone. Niobium and titanium precipitates of different morphology and size range evolved during thermomechanical processing and include rectangular (∼500 nm), irregular (∼240–500 nm), cuboidal/spherical (∼125–300 nm), and very fine (

Microstructure and high strength–toughness combination of a new 700 MPa Nb-microalloyed pipeline steel

In modern industry, the developing tendency and prospect for productions of the oil and gas pipeline steels is to further improve the strength and toughness by advanced manufacture process of the thermo-mechanical control process (TMCP) to refine microstructure. In this work, the hot deformation behavior as well as its effect on the phase transformation of the clean acicular ferrite pipeline steel with simple chemical composition has been investigated. According to the result, the optimum TMCP parameters were designed. Furthermore, the rolling test was carried out on the experimental rolling mill. The results show that, the high strength and excellent toughness of the clean acicular ferrite pipeline steel can be obtained by controlling the TMCP parameters of the production process appropriately. (c) 2006 Elsevier B.V. All rights reserved.

Challenge of mechanical properties of an acicular ferrite pipeline steel

In the present study, comparison studies on mechanical properties were made on a commercial X70 grade polygonal ferrite (PF) dominated pipeline steel and a laboratory developed X90 grade acicular ferrite (AF) dominated pipeline steel obtained by optimum thermo-mechanical controlled processing (TMCP). Charpy impact test results indicated that the upper shelfenergy (USE) of the AF pipeline steel was a little bit higher, but its energy transition temperature (ETT) was extremely low, about -162 degrees C, much lower than that of the PF pipeline steel of about -121 degrees C. It was analyzed that higher strength and better toughness of the AF pipeline steel came from its finer grain size and higher density of dislocations and subboundaries, which could be also further proved from the electron backscatter diffraction (EBSD) analysis for its finer effective grain size (EGS) and higher content of low angle grain boundaries (LAGBs), smaller cleavage fracture unit measured from the fracture surface around fracture origin fractured at -196 degrees C, and its more bent crack propagation path in the fracture. (C) 2008 Elsevier B.V. All rights reserved.

Study of high strength pipeline steels with different microstructures

Transformation behavior and morphological characteristics of acicular ferrite in linepipe steels were investigated through the use of a dilatometer and EBSD. The results show that the volume fraction of acicular ferrite increased with an increase in the amount of hot-deformation in the austenite non-recrystallization region because acicular ferrite is formed at nucleation sites such as dislocations within austenite grains. This study found that acicular ferrite is formed at approximately 600 °C, where the transformation behavior can be characterized as a two-stage reaction : (i) nucleation of acicular ferrite and (ii) formation of polygonal ferrite between acicular ferrite grains. EBSD analyses show that an acicular ferrite grain consists of several sub-units misoriented by 1–2° and that a set of adjacent acicular ferrite grains with crystallographic misorientation below 15° makes up the crystallographic packet .

Transformation behavior and microstructural characteristics of acicular ferrite in linepipe steels

The paper describes the microstructural constituents in a industrially processed Nb-microalloyed pipeline steel having yield strength of ∼620 MPa. The microstructure of base, heat affected zone (HAZ), and weld metal of the fabricated steel pipe was examined by optical and transmission electron microscopy. The microstructure of thermomechanically processed pipeline steel primarily consisted of non-equiaxed ferrite of mixed morphologies with small fraction of degenerated pearlite. The microstructure contained high dislocation density, sub-boundaries and dislocation substructures. The HAZ was characterized by a combination of fine and coarse grained polygonal ferrite structure with high density of dislocations and fine cementite particles. In the weld metal, the constituents of complex ferrite were low temperature transformation products formed during continuous cooling such as quasi-polygonal or massive ferrite, acicular ferrite, bainitic ferrite and dispersion of coarse and fine cementite particles in the ferrite matrix. The precipitates in the investigated pipeline steel were of duplex type containing either Nb and Ti or Ti and Mo, even though the steel contained low concentration of titanium. Precipitates of different morphology and size range were observed and include rectangular (∼100–130 nm), cuboidal/spherical (∼20–100 nm), fine (∼10–20 nm) and very fine (

Microstructure of high strength niobium-containing pipeline steel

Acicular ferritic microstructure of a low-carbon Mn–Mo–Nb microalloyed pipeline steel

The effects of alloying elements in welding wires and submerged arc welding process on the microstructures and low-temperature impact toughness of weld metals have been investigated. The results indicate that the optimal contents of alloying elements in welding wires can improve the low-temperature impact toughness of weld metals because the proeutectoid ferrite and bainite formations can be suppressed, and the fraction of acicular ferrite increases. However, the contents of alloying elements need to vary along with the welding heat input. With the increase in welding heat input, the contents of alloying elements in welding wires need to be increased accordingly. The microstructures mainly consisting of acicular ferrite can be obtained in weld metals after four-wire submerged arc welding using the wires with a low carbon content and appropriate contents of Mn, Mo, Ti-B, Cu, Ni, and RE, resulting in the high low-temperature impact toughness of weld metals.

Effects of welding wire composition and welding process on the weld metal toughness of submerged arc welded pipeline steel

A sintering welding wire for the multi-wire high-speed autoamtic hidden arc welding contains MgO (19-28 mass%), CaF2 (12-18%), ZrO2 (8-15%), Al2O3 (18-24%), SiO2 (17-25%), CaO (7-13%), RE (0-7%), MnO(0-5%), TiO2 (0-5%), Na2O+K2O (2-5%), S (0-0.05%), and P (0-0.06%). Its advantages are stable arc, beautiful welded seam, and high welding speed (1.5-2 m/m).

De-liang Ren

Papers

Acicular ferritic microstructure of a low-carbon Mn–Mo–Nb microalloyed pipeline steel

Effects of welding wire composition and welding process on the weld metal toughness of submerged arc welded pipeline steel

Sintered flux utilized in automatic submerged arc welding with multiple wires in high speed