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Info Connect | September 2014

Development and production of high strength pipleline steel

The predicted growth in energy consumption in the coming decades necessitates severe efforts for transporting large amounts of natural gas to the end user. Large-diameter pipelines are the best and safest means of transport. The high cost of natural gas forces the pipeline operator to explore all the possibilities of reducing the cost of a pipeline project in the future. The steel manufacturer can assist by supplying high quality steel.  

For higher transportation efficiency, the trend in the pipeline design is to use larger diameters with higher operating pressures. This results in a requirement for steels with high yield and tensile strengths. The operation of pipelines in arctic regions, and/or the transportation of liquid natural gas (LNG) also demand high material toughness at low operating temperatures.

 

A particularly important trend has been the exploitation of sour oils and gas reserves, which has required the development of linepipe steels with resistance to hydrogen-induced cracking (HIC). Thus in addition of higher strength and toughness, the pipeline technologies require improved resistance to corrosion to be met with specific alloy additions and special control over non-metallic inclusions.

 

Both welded and seamless pipes are used in pipeline construction, but as the welded pipes can be made in larger diameters than the seamless pipes, the high capacity pipelines are essentially made from the welded pipes. Both longitudinal and spirally welded pipes are used in the line pipe industry. The welding operations thus require that the materials should be weldable not only during pipe manufacturing process but also at the pipe laying sites where the control over welding conditions becomes difficult. All these requirements, over the years, are met through the provision of steels with progressive increase in yield strength, high weldability and sufficient toughness to restrict crack propagation particularly at low operating temperatures. While the selection of appropriate chemical composition is important to meet the requirements, the thermomechanical processing helps to achieve the specific property requirements. In the case of off shore pipelines, the operating pressure is not the most important but rather the ambient water pressure. Therefore, one of the design criteria for offshore pipelines is less the strength but more the collapse behaviour of the pipe. The pipe to be used in offshore pipeline construction should possess not only good materials properties but also good geometry to ensure good collapse strength. As the H2S content of the gas being transported increases, the requirements for HIC resistance of the pipe material increase. When an aqueous phase is present, CO2, HS and chlorides are extremely corrosive. For applications in such corrosive environments, a pipe made of either all-corrosion resistant material or of a low-alloy steel pipe clad with a high-alloy corrosion resistant material is used The most popular document specifying the linepipe material is the American Petroleum Institute (API) standard API 5L [API, 2000] that specifies steel grades with a range of properties that cover both the high test linepipe and spiral weld linepipe steels.

 

In the seventies, the hot rolling and normalising was replaced by thermomechanical rolling. The latter process enables materials up to X70 to be produced from steels that are microalloyed with niobium and vanadium and have reduced carbon content. X70 and beyond, possess highly refined grain and high cleanliness. They are characterized by the low sulphur content and reduced amount of detrimental second phases such as oxides, inclusions and pearlite.

 

An improved processing method, consisting of thermo mechanical rolling plus subsequent accelerated cooling emerged in the eighties. By this method, it has become possible to produce higher strength materials like X80, having a further reduced carbon content and thereby excellent field weldability. Additions of molybdenum, copper and nickel enable the strength to be raised to that of grade X100, when the steel is processed to plate by thermo mechanical rolling plus modified accelerated cooling.

 

JSW Steel is one such steel manufactures which contributes to reducing operational costs of pipeline over its life by investigating the various required parameters for high strength steel. JSW Steel is one of the pioneers in manufacturing API grade in the country. At our Vijayanagar manufacturing facility the company is in relentless pursuit of technological advancement, unwavering focus on innovation, strident emphasis on quality to further raise the bar for API grade. These constant initiatives enabled the company to manufacture X70 for the need of pipe manufacturers and have lead us to gain confidence of leading API Pipe grade pipe manufactures like Welspun Corps ltd, Jindal SAW, Ratnamani Metals & Tubes Ltd, Man Industries and Maharashtra Seamless Ltd.

 

Latest developments:

 
     
  • High strength steel grades are showing improvements in mechanical properties. The limitations on strain capacity were recognized and are being properly addressed;
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  • Increase of strength is associated with increasing Mo, Si and Ni content;
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  • Independent of the variations in the alloying element content, the characterizing parameters for good weldability (i.e. CE and Pcm) are maintained practically unchanged;
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  • Development of optimum microstructures (e.g. ferrite-bainite DP and/or bainite-martensite/austenite) which provide the required mechanical properties for high strain capacity applications, such as higher strain hardening and uniform elongation;
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  • Both ´conventional´ and ´unconventional´ TMCP processes showed to be effective processing routes in order to produce steels with lower Y/T ratio and higher em, and sufficient toughness.

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