Development of Rolled H-shape Steels for Low Temperature Applications

Abstract In recent times, demand for steels, such as H-shaped steel for offshore structures for the world energy market is rapidly increasing. Such beams are often manufactured with steel plates and are welded to ensure the toughness, which is represented with Charpy V-notch energy, at low temperatures of -40 °C in H-shape steel. However, welded beams have several disadvantages, e.g., requirements of cost and operation period for welding fabrications and inspections and quality control in heat affected zones. Rolled H-shape steels enable to overcome these difficulties if they satisfy the mechanical property requirements such as Charpy V-notch absorbed energy at -40 °C. In previous studies, grain refinement by facilitating intragranular ferrite nucleation on inclusions in steels has been analyzed to improve strength and toughness. For example, vanadium-nitride is reported to be particularly capable as ferrite nucleation sites. Furthermore, precipitates and inclusions can be useful for rolled H-shapes to improve the toughness by the refinement of ferrite grains. Chemical composition, particularly of microalloying elements, which form precipitates and inclusions, has to be carefully optimized to satisfy alloy ranges identified in EN10225 classification. This study attempts to reveal the effects of microalloying elements in H-shapes for the low temperature applications. Series of experimental ingots were casted in a vacuum melting furnace and were laboratory rolled to plates. These test materials were investigated by mechanical tests and metallographic observations such as optical and transmission electron microscopy. The results of these experiments suggested that the toughness can be improved by the ferrite refinement using vanadium-containing nitrides on prior austenite grain boundaries. Based on these experimental results, actual H-shapes were examined in the practical production line and it was confirmed that these results could meet the EN10225 Class S355 classification, where the Charpy V-notch minimum average impact energy at -40 °C required were 50 J in base metals and 36 J in weld heat affected zones.