An experimental HSLA steel was produced by the electric arc furnace, vacuum degassing, ladle treatment and continuous casting route. The experimental steel was then rolled in a laboratory using a hot rolling schedule to simulate an industrial controlled hot rolling procedure for the production of plates as closely as possible to investigate the effect of a thermomechanical processing schedule plus the use of water quench, accelerated cooling followed by forced nitrogen gas or air as cooling media, on the mechanical properties of plates. The results showed that the controlled thermomechanical hot rolling schedule of slabs followed by the cooling of plates in either forced nitrogen gas or by accelerated cooling exhibited target properties equivalent to a steel grade API X-80.
to maintain a high initial temperature so that finishing roughing passes are completed at fully austenitic temper-An API 5L X-70 grade steel for large diameter ature and to ensure the dissolution of carbides, nitrides, or pipeline application with sour gas resistance was carbonitrides which need to be precipitated at some later developed. The resulting Fe-C-Mn-Nb slabs were stage of processing.7 When the steel is deformed at high controlled rolled and accelerated cooled. In most of temperatures, recrystallisation will be complete before the slabs, mechanical properties equivalent to the subsequent deformation steps. Following recrystallisation, above grade were broadly achieved. However, grain growth can occur, depending on time, temperature, despite achieving excellent mechanical properties, and amount of precipitates. As the temperature decreases, during the test period, several slabs exhibited recrystallisation can be partially or totally suppressed, so cracks in the cooling yard, causing their rejection.that grain flattening and strain accumulation occurs. To elucidate the nature of the crack, severalAt very low temperatures, the deformation will be specimens were analysed by employing optical, extended to the austenite plus ferrite region and both scanning electron, and transmission electron phases will be deformed. Precipitate compounds will appear microscopy. Microstructural characterisation as the temperature of the steel decreases, supersaturating together with microanalysis and thermal analysis the austenite so that precipitate nucleation occurs, followed carried out on as cast specimens showed the by particle growth.8 Accelerated cooling practice after presence of rodlike and/or dendritelike precipitates the final rolling will ensure the strength properties by of the Fe 2 Nb type associated with the cracks. controlling temperature and microstructure,9 with the The elimination of the cracks was achieved by objective of forcing deformation and recrystallisation increasing the niobium content in the Nb ferroalloy to lower temperatures and thereby maximising austenite to ensure its dissolution in the liquid bath after its grain refinement;10 of course, the choice of technique used addition during the alloying stage.I&S/1416 will depend on the microalloying system in use and the particular product form and manufacturing facilityEng. Mendoza, Eng. Huante, and Eng. Alanis are with Ispat available.
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