Yoshio Terada

Effects of Microstructure and Texture on DWTT Properties for High Strength Line Pipe Steels

The crack arrestability for high strength line pipe steels with tensile strength of 650 to 850 MPa was evaluated using precrack DWTT (pc-DWTT). Moreover, the effects of microstructure and texture on pc-DWTT energy were investigated. The pc-DWTT energy was remarkably affected by tensile strength. The pc-DWTT energy of ferrite and bainite/martensite dual phase steels was much higher than that of bainite single phase steels in comparison with the same tensile strength. The {100} plane is a cleavage plane in iron, so the brittle crack mainly propagates along the {100} plane. Bainte single phase steels indicated a high intensity of the {100} on the plane rotated 40° from the rolling plane with the axis of the rolling direction. On the other hand, ferrite and bainite/martensite dual phase steels indicated not only a high intensity of the {100} plane rotated 40° from the rolling plane, but also a high intensity of the {100} plane parallel to the rolling plane. Slant fracture could be easily formed by the high intensity of the {100} on the plane rotated 40° from the rolling plane if local brittle areas such as martensite and austenite constituent (M-A constituent), which became the initiation point of brittle fracture, existed. In contrast, separation tended to be formed by the high intensity of the {100} plane parallel to the rolling plane that was caused by the formation of ferrite and bainte/martensite dual phase microstructure. Thus, pc-DWTT energy and shear area were remarkably affected by microstructure and texture. Therefore, to control microstructure and texture is vay important for the improvement of pc-DWTT properties.Copyright

Change of Mechanical Properties of High Strength Line Pipe by Thermal Coating Treatment

In strain-based design, the overmatch condition in the girth weld portion primarily must be maintained. The pipes may also be required to have a low yield to tensile (Y/T) ratio and a high uniform elongation (U.EL) in the longitudinal direction to achieve a high compressive buckling strain. However, change in the mechanical properties by heating during coating treatment has not been paid attention so much. Furthermore, how much the mechanical properties change is affected by production conditions is unclear. This study aims to clarify firstly the relation between the mechanical properties (Y/T ratio, U.EL etc.) and the microstructure and secondly the change in mechanical properties by thermal coating treatment. The Y/T ratio and U.EL are affected by the volume fraction of ferrite and the secondary phase, which are changed by thermomechanical control processing (TMCP) conditions. For example, use of dual phase microstructure is very effective for decreasing the Y/T ratio and increasing the U.EL as the pipe. On the other hand, yield strength (YS) rises and the U.EL does not change after coating. The increase in the YS after coating is influenced by the microstructure and TMCP conditions. Resultantly, dependence of the Y/T ratio on the microstructure and TMCP conditions is reduced for line pipes after thermal coating treatment.Copyright

X100 Linepipe With Excellent HAZ Toughness and Deformability

Good low-temperature toughness of the base material (BM) and weld heat-affected zone (HAZ), and good deformability of the pipe body together with good field weldability are required for X100 linepipe to ensure the safety of pipelines and to facilitate field welding. It is, however, very difficult to attain these properties simultaneously because of the large addition of alloys. The technology of improving HAZ toughness by reducing carbon content through the reduction of M-A constituents harmful to low-temperature toughness was developed, and accelerated cooling after controlled rolling was applied to attain good low-temperature toughness of BM and high uniform elongation together with sufficient strength corresponding to X100. Two newly developed types of X100 linepipe, a “high HAZ toughness type” and a “high uniform elongation type”, exhibited excellent low-temperature toughness of the HAZ and high uniform elongation together with sufficient strength, respectively.Copyright

Development and Properties of Ultra-High Strength UOE Linepipe

High-pressure operation through high strength linepipe reduces long distance transportation cost of natural gas. In order to maximize the cost reduction, X120 UOE pipe has been developed. Low C-Mo-B steel with fine-grained lower bainite (LB) microstructure realizes high strength, excellent low temperature toughness and good weldability. The technology was verified in small-scale commercial production or “mini-rolls”. Suitability for use as linepipe was demonstrated through an extensive development program that covers burst test, fracture toughness evaluation, girth welding technology, etc. A demonstration line was successfully constructed using the pipes manufactured in the “mini-rolls”.Copyright

Application of B-Added Low Carbon Bainite Steels to X80 UOE Line Pipe for Ultra Low Temperature Usage

Demand for high strength line pipes is increasing because of the reduction in natural gas transportation costs of pipelines. Low temperature toughness is required for high strength line pipes. Reduction in manufacturing cost of high strength linepipes is also required in an environment where alloying cost is increasing. To meet these requirements, boron (B) addition is extremely useful because the addition of very small amounts of B remarkably improves the strength and low temperature toughness. B-added low carbon bainite (LCB) line pipes with American Petroleum Institute (API) grade X60 to X80 have been developed for several decades [1–2]. B-added LCB steels have excellent low temperature toughness, however, it is challenging to achieve excellent crack initiation resistance and crack arrestability for ultra low temperatures such as −60°C. In particular, it is very difficult to achieve both excellent Drop Weight Tear Test (DWTT) properties of base metal, and excellent Charpy V-Notched (CVN) properties of seam welds in heavier wall thickness of X80 UOE linepipe. Metallurgical concepts such as the optimum chemical compositions, Thermo Mechanical Control Process (TMCP) conditions and seam weld conditions of B-added LCB steels with API grade X80 for ultra low temperature have been proposed in order to achieve the excellent mechanical properties even in a low manufacturing cost. Based on this concept, excellent DWTT properties of base metal and CVN properties of the seam welds of API grade X80 line pipe with 25mm thickness down to –60°C were obtained.Copyright

X120 UOE Linepipe With Improved Properties and Varied Sizes

Nippon Steel has developed X120 UOE linepipe as a part of joint development program with ExxonMobil. After establishment of the basic production technology of X120, the mechanical properties have been improved, especially to increase the suitability to strain based design application and the size range has been expanded. The combination of heavily deformed austenite grains and a mild accelerated cooling to lower cooling stop temperatures (MAC) achieves the aim to lower the yield strength (YS) and the yield to tensile (Y/T) ratio for the B-containing chemistry that was decided to meet the CG-HAZ toughness target. The properties of MAC pipes met the targets and the pipes with OD from 28” to 48” and WT from 13mm to 20mm were made for test purpose.Copyright

High strength UOE pipe with excellent CTOD properties and deformability

New steel manufacturing technology has been developed to attain excellent HAZ toughness of high-strength steels. In this steel, the HAZ microstructure near a weld fusion line is refined markedly by utilizing the strong retardation of austenite grain growth as well as the formation of intragranular ferrite. A dual phase microstructure is very effective for obtaining both high uniform elongation and low Yield / Tensile ratio in the high strength UOE pipe body. High strength, low Y/T ratio and high uniform elongation of the steel pipe can be attained by optimizing the area fraction of ferrite and the grain size by applying accelerated cooling technology. New high strength UOE pipes called “Tough-Ace” possessing both excellent HAZ toughness and deformability have been developed, and the X60 UOE pipe has been mass-produced for the Sakhalin Project.Copyright

Metallurgical Design and Development of High-Grade Line Pipe

The application of high-strength line pipes has enabled pipelines to operate at high pressure, generating cost savings for both gas transportation and construction. In general, high-strength line pipes require crack initiation resistance and crack arrestability at low temperatures, as well as field weldability. High strength and deformability for strain-based design and excellent sour resistance are also required. Moreover, composite properties are often required for high-strength line pipes. This paper describes our progress in this field with regard to metallurgical design and development. Metallurgical design aimed at achieving a good balance between strength, low temperature toughness and deformability for strain-based design is also described from the perspectives of grain refinement, microstructure and chemical composition. Metallurgical design focused on a good balance between strength and sour resistance in limited low chemical composition is described from the perspectives of microstructure and control to chemical composition and center segregation. These efforts have led to the development of high-strength heavy wall line pipes of API X60 to X100 grades offering excellent low temperature toughness and high deformability for stain-based design, while API grades X65 to X70 with good sour resistance have also been developed.Copyright

X80 UOE Pipe With Excellent HAZ Toughness

New X80 UOE pipe manufacturing technology has been developed to attain excellent CTOD properties in HAZ. In the new steel pipe, the HAZ microstructure near the weld fusion line is refined by utilizing the strong suppression of austenite grain growth as well as the formation of intragranular ferrite (IGF). The IGF grows radially from oxides in the same way as in Ti-O steel. The suppression of austenite grain growth is due to the pinning effect by fine particles including ultra fine oxides. As these oxides dispersed in the steels are chemically stable even near the fusion line, the microstructure can be refined. In addition, it is also necessary to suppress the formation of coarse grain boundary ferrite (GBF) near the weld fusion line in order to obtain excellent CTOD properties. So, it was found that the increase of boron addition in weld metal is effective to suppress the formation of coarse GBF near the weld fusion line because boron atoms diffuse into prior austenite grain boundaries near the fusion line from the molten pool during welding. This paper describes new technology for improving CTOD properties in HAZ. The X80 UOE pipe manufactured on a large scale and exhibited excellent CTOD properties at −30°.Copyright

Metallurgical Design and Development of High Deformable X100 Line Pipe Steels Suitable for Strain-Based Design

The demand for natural gas using pipelines and LNG to supply the world gas markets is increasing substituting for oil and coal. The use of high strength line pipe steels provides the reduction of cost of gas transmission pipelines by enabling high-pressure transmission of large volumes of gas. In particular, high strength line pipe materials with a yield strength of X80 or higher have been developed over the last few decades around the world. Long distance gas transmission pipelines from remote areas sometimes pass through discontinuous permafrost, and are subject to ground movements by repeated thaw subsidence and frost heave. In this case, strain-based design has been applied as well as stress-based design. Therefore, high deformable line pipe is required for strain-based design in order to prevent the pipeline from fracturing. Nippon steel has also developed high deformable high strength line pipe material suitable for strain-based design. In recent years, demand for high strength line pipe steels has emerged in which the molybdenum content is reduced because of the high cost of molybdenum. Conventionally, high strength line pipe steel with Mo addition has been developed in order to control the microstructure and to obtain pipe properties such as strength and low temperature toughness. This paper describes the metallurgical design and development of high deformable high strength X100 line pipe with lower Mo content suitable for strain-based design. High deformable X100 line pipe with 16 mm wall thickness as well as good low temperature toughness and seam weld toughness has been developed.© 2008 ASME