New Generation Structural Steel Plate Metallurgy for Meeting Offshore and Arctic Application Challenges

The new generation of value-added low carbon-low manganese-niobium microalloyed structural steels for both low and high yield strength, energy absorption, fatigue and fracture resistant applications is under development for offshore and arctic materials engineering applications. These materials engineering considerations are shifting designers to consider new lower cost and more robust construction materials even for low yield strength applications require improved fatigue, fracture arrest and toughness performance. The civil engineering and end user community demand structural reinforcing bars, shapes, beams and plates with improved energy absorption and fatigue properties. With more severe climatic conditions evolving every day, demands also necessitate improved fire and seismic resistance, yield-to-tensile ratio consistency, improved bendability and weldability. These attributes are difficult to obtain from steel producers today with their current higher carbon microalloyed steel approaches and hot rolling practices. There is a global shift in motion to low C-Nb-Mn bearing construction steels displacing traditional materials. The technological and metallurgical advancements of value-added niobium (Nb) microalloyed thermo-mechanical controlled process (TMCP) plate steels have been further developed to meet more demanding fatigue, fracture and low temperature toughness end user requirements. Niobium enables achievement of substantial grain refinement and grain size uniformity when the plate is rolled with the proper reduction, thermal schedule and process metallurgical operational practices. The effects of microalloying elements on the continuous cooling transformation behavior must be carefully controlled during the process metallurgy of the reheating and rolling process to successfully achieve the desired mechanical properties. TMCP applications have been successfully developed in numerous product sectors with thickness exceeding 120 mm. Since the very fine grained microstructure improves toughness and increases the yield strength, this Nb-TMCP process enables the required tensile properties with the growing trend to leaner chemical composition designs (less than 0.10%C) and excellent toughness properties. From an operational cost perspective, in today’s very competitive market environment, there exists a huge opportunity for structural offshore and arctic plate producing steel mills to improve their profitability by thoroughly assessing a shift to lower carbon and manganese steels in their product mix. Through the adoption of these lower carbon Nb-containing structural materials, several design and/or manufacturing companies are initiating new offshore steel designs that will further provide improved overall lifetime and cost performance at reduced maintenance expense. These high strength plate steels offer the opportunity to manufacture complex heavy-lift and fatigue-critical components for larger offshore structures without increasing the weight of the platforms.