Juan Garcia de Blas

Development of controlled rolled high strength Nb and Nb-V strip steels for X70 ERW pipes

An increasing market need for high strength low alloy (HSLA) strip steels for oil and gas API (American Petroleum Institute) pipelines of superior grades has placed an even increasing pressure upon the steel works for high quality steel strip.In order to achieve this goal as well as to optimize the controlled rolling process of its hot strip mill, COSIPA Steel Co. and the Federal University of Rio de Janeiro (COPPE/UFRJ) joined together in a research program. The aims of this program consisted in developing 6.35 and 9.52 mm hot strip steels which fulfilled the API 5L X70 requirements and could be characterized in terms of mechanical and impact toughness properties, and the optimization of previously developed HSLA strip steels for lower API grades.The alloys design was based on Nb and Nb-V alloyed steels. The hot strips were thermomechanically processed with control of operation parameters such as the reheating temperature, the finishing temperature, the coiling temperature, and the rolling schedules.The sampling of the steel produced was carried out along the strip length so that they could be fully characterized in terms of microstructure, and mechanical and Charpy tests, either as strip or the resulting electric resistance welding (ERW) pipes.The results make evident the relation between mechanical properties, microstructures, and process conditions.