Estudo da influência de variáveis de laminação em uma linha de tiras a quente convencional nas propriedades mecânicas de um aço alto nióbio da classe API 5L X70

Abstract

The needs for high- and ultrahigh-strength steels utilization in pipe fabrication for energy pipeline construction has demanded for continuous development of alloy design alternatives, which present a good weldability and high toughness at low temperatures. In order to reach the desired balance between such properties, the use of alloys with low carbon equivalent and a refined microstructure has turned imperative. In this direction, the development of steels containing low carbon, high manganese and high niobium has received major attention during the past 15 years. This alloy design conception is called High Temperature Processing (HTP) Steel, since it turns possible to suppress static austenite recrystallization even when straining is made at higher temperatures than those employed to commercial HSLA steels in flat hot rolling lines. This study has screened the influence of two thermomechanical processing variables over the microstructure and the mechanical properties of API 5L X70-PSL2 coils with chemical composition typical of a HTP steel, manufactured by Usiminas Ipatingas Hot-Rolling Strip Mill. It has been verified that, for the processing conditions employed, there were not relevant changes on the resulting microstructure, which was constituted by a very fine polygonal ferrite added to a lower volumetric fraction of second phase, constituted by degenerated pearlite (between 5 to 10%) and other carbides. The average each sample FGS measured from 3.9 to 5.3 µm. Mechanical parameters (YS and UTS) and elongation stayed well above the minimum limits required for ERW pipe fabrication for the specified steel grade. Maximum encountered hardness, of 223 HV10 has not exceeded the maximum value specified for this pipe category. Fracture toughness, measured by impact Charpy test, has shown an approximated transition ductile-fragile temperature equal to -60°C. Reducing C content and not adding V, as typical in HSLA steels, a reduction up to 0.04% in the Ceq value, as calculated by IIW formula, turning evident a potential weldability improvement for this steel grade. Acicular constituents formation has not been possible to be reached for both, the deformation amount and the cooling rates applied in this experiment.