The HSLA (High-Strength Low Alloy) steels are used in the production of pipes, flanges and connectors to build ducts for ore, oil and gas transport. The conventional processes are the rolling or forging. In the transport of ore and heavy oil, the abrasive particles impair the surfaces and reduce the pipelines lifetime. Therefore, besides the mechanical properties as API 5L, it is important to verify the wear resistance of these steels. In this context, two microalloyed steels were forged in the form of square bars. Thereafter, specimens of these bars were annealed at 930 ºC, quenched at 900 ºC and tempered at 600 ºC. Tensile and wear tests were performed. The results show that molybdenum and niobium present similar effects on phase transformation of steels, promoting a desired acicular ferrite/bainite microstructure and fulfill the mechanical strength of API 5L-X70 standard. The molybdenum has dominating effect in the hardenability when in solid solution, however, after tempering, thermodynamic simulation by FactSage software indicates that niobium probably promotes secondary hardening.

RESUMO Nos aços microligados da série API (American Petroleum Institute) utilizados na fabricação de dutos de grande diâmetro, a resistência mecânica deve ser suficiente para suportar as elevadas pressões do fluido transportado na indústria petroquímica. O aumento da resistência mecânica e da tenacidade para permitir a operação da linha em pressões mais elevadas, sem a ocorrência de colapso da estrutura, é o principal requisito para os aços utilizados nestes dutos. No Brasil, as empresas produzem estes aços por meio da laminação de tiras a quente ou chapas grossas, com adição de nióbio, titânio e vanádio, conforme os requisitos da norma API 5L. Considerando a complexidade geométrica dos componentes utilizados na indústria petroquímica, tais como flanges e centralizadores de tubos, é importante desenvolver um processo alternativo para fabricação de acessórios com as exigências desta norma. Dentro deste contexto e com o objetivo de avaliar novas técnicas de processamento, foi elaborado um aço com composição química similar à de um API 5L X70. O metal líquido foi vazado em uma lingoteira e forjado em barras com secção quadrada. As barras foram recozidas e amostras foram cortadas para confecção de corpos de prova que foram submetidos à têmpera e revenimento em diferentes temperaturas. As análises microestruturais e propriedades mecânicas de dureza e tração, após os tratamentos térmicos, foram comparadas às existentes na literatura para tubos de aços laminados. Os resultados mostram que as condições dos tratamentos térmicos são importantes para desenvolver uma rota alternativa de forjamento controlado, para aços com características da norma API 5L.

ABSTRACT In API (American Petroleum Institute) microalloyed steels used in the manufacture of large diameter pipelines, the mechanical properties must be sufficient to withstand the high pressure of the transported fluid in the petrochemical industry. The increased of yield strength and toughness to allow operation at higher line pressure, without collapse of the structure, is the main requirement for these steels. In Brazil, industries produce these steels by hot rolling of strips or plates, with the addition of niobium, titanium and vanadium, as the requirements of API 5L standard. Due to the geometric complexity of the components, such as flanges and pipes connections, it is important to develop an alternative process for the manufacture of these accessories. Regarding this context and in order to evaluate a new processing, one steel was developed with chemical composition as an API 5L X70. The liquid metal was poured into an ingot mold and forged bars were manufactured with square section. The bars were annealed and samples were cut. The samples were submitted to quenching and tempering at different temperatures. Hardness measurements, microstructural analysis and mechanical properties, after quenching and tempering treatments were compared with literature results for the rolled steel tubes. Preliminary results show that the heat treatments conditions are important to develop an alternative route by controlled forging for steels, with standard API 5L characteristics.

Abstract This study evaluated the influence of plastic deformation during hot strip rolling in the size of TiN precipitates in an industrial Ti microalloyed dual phase steel. TiN is usually used for pinning austenitic grain boundaries during recrystallization and its efficiency depends on factors such as size of the precipitates and Ti precipitated fraction. During the slab reheating process, there occurs a partial dissolution of TiN precipitates that subsequently reprecipitates during the hot rolling process. However, this reprecipitation preferentially takes place at crystalline defects which act as nucleation sites. These sites increase with the amount of applied plastic deformation, resulting in smaller precipitates, due to the partial dissolution of larger particles or due to reprecipitation in the nucleation sites. Results indicate a reduction of about 28 % in the average size of the precipitates for samples extracted from hot bands (with deformation) and, therefore, a significant reduction in austenitic grain size, when compared with samples extracted from slabs (with no deformation).