D. Satish Kumar, S. Manjini, P. K. Ghorui, P. C. Mahapatra and Madhu Ranjan:
Development of critical properties in line-pipe steels
Line pipe steels, micro-alloying, YS/UTS ratio, rolling regimes
Transportation of oils and gases through pipe lines is the most economic and rapid mode being employed globally. Linepipe steels were developed to meet the stringent requirements with respect to strength, toughness, and weldability. Gas transportation becomes more critical due to service leakages and condensation/freezing of gas under low temperature condition and makes the low temperature properties more stringent. One important criterion for these steels is to maintain low-temperature toughness and a low yield strength to tensile strength ratio (YS/UTS) of 0.85 max. Combination of high tensile properties and low temperature impact strength is controlled by optimal microstructures of steel, which is obtainable with the determining effect of microalloying and regimes of rolling. With a view to meet these characteristics, studies were carried out with varying carbon contents in combination with micro-additions of Nb/V/Cr/Ni/Ti to produce hot rolled coils of strength equivalent to API X60 with low YS/UTS ratio and API X65 with excellent low temperature toughness. Low YS/UTS ratio was achieved in a dualphase tanium microalloying. Rolling parameters were optimized by processing at different thicknesses to examine the variation in properties. Chemistry structure property relationships developed through these trials were utilized in developing steels having strength equivalent to X70 grade steel with minimum number of trials. Chromium based chemistry were found to match all the specifications. The paper addresses various issues of al-loying, rolling and cooling to arrive at optimum structure and properties in line-pipe steels for specific applications.
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