Processamento de um aço ARBL no laminador de tiras a quente da CST visando atender a norma API 5L, Grau X70

Abstract

Companhia Siderúrgica de Tubarão (CST) decided to start the production of hot rolled coils for gas and oil pipelines, after the commissioning of its hot strip mill. There is a demand for the application of high strength steels in these pipelines, allowing a decrease in pipe wall thickness without affecting the pipeline operation pressures. Considering this situation, CST started the development of hot rolled coils for API 5L - X70 grade. The development of the API 5L - X70 steel was based on HSLA steel, microalloyed with Nb, V and Ti. Nine hundred tons of this material were hot rolled under controlled thermomechanical conditions, involving the control of the finishing and coiling temperatures. This allowed the use of grain refinement, precipitation hardening andphase transformations in order to obtain the required microstructures and mechanical properties. The use of the above conditions led to the development of API 5L - X70 hot rolled steel plates 9,47mm thick. There was an increase in the mechanical strength of the steel as the finishing temperature was lowered. No increase in the mechanical strength of thematerial was caused by the lowering of the coiling temperature, despite the observed increase in the volume fraction of acicular ferrite in the material as the coiling temperature was lowered.