Demand for high strength line pipes is increasing because of the reduction in transportation costs of pipelines. The development of high strength with cost effectiveness is also required in an environment where alloying cost is increasing. Low temperature toughness is required for high strength line pipes. 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 API (American Petroleum Institute) grade X60 to X80 have been developed for several decades. The B-added LCB steel had good toughness, but it was difficult for the steel to improve the DWTT (Drop Weight Tear Test) properties, which are required for crack arrestability. In this study, the optimum chemical compositions and Thermo Mechanical Control Process (TMCP) conditions to improve DWTT properties were investigated. Excellent DWTT properties of B-added LCB steel with a thickness of 25mm down to -60oC were obtained. In addition, simulated CGHAZ (Coarse Grained Heat Affected Zone) toughness for this steel was improved down to -60oC.
Recently, the application of high strength line pipes with API grade X80 or higher is increasing for transportation efficiency of natural gas under high-pressure operation. The demand for long distance gas pipelines from the arctic area to a large consuming region is also increasing. Generally, crack initiation resistance and crack arrestability are required for high strength line pipes. In the arctic area, these properties down to -60oC for base metal and seam welds are required for the application of pipelines. The price of alloying elements such as molybdenum (Mo) and nickel (Ni) has soared recently. The addition of these alloying elements to obtain high strength causes the increase of the material cost.
