Process Metallurgy & Product Applications
The possibilities of using titanium-alloyed grades for the production of high-strength, low-alloy steels are widely described in scientific articles. However, the practical usage is difficult, since many production factors significantly influence the strengthening mechanism and structure formation of titanium-alloyed grades. This paper describes the influence of various technological factors on the properties of titanium-alloyed steels produced by the Compact Strip Production process.
Direct quenching and accelerated cooling (DQ-AcC) at Nucor Steel Tuscaloosa Inc. has been used to produce API 5L X70M skelp successfully for both straight- and spiral-seam welded pipe mills at Welspun Tubular LLC. Compared to the conventional laminar flow cooling that produces ferrite-pearlite microstructure, DQ-AcC results in uniform acicular ferrite and upper bainite through-thickness, which enhances strength and toughness significantly. To understand the losses of yield strength and toughness due to strain softening, mechanical behavior shifting of API 5L X70M skelp through accelerated cooling and spiral forming processes is examined.
The mechanical properties of steel are influenced by grain size, which can change through mechanisms such as nucleation and growth at elevated temperatures. However, the classic Johnson-Cook model that is widely used in hot deformation simulations does not consider the effect of grain size on flow stress. In this study, the Johnson-Cook model was modified to incorporate the effects of austenite grain size on flow stress. A finite element model was employed to characterize the effects of grain size on the flow stress for different steel grades over a range of temperatures (900°C to 1,300°C). Simulation results show good agreement with experimental observations.
End user in-service demands are becoming increasingly challenging in customer specifications for heavy plates specified in pressure vessels for petrochemical, chemical and cryogenic reactors. New-generation value-added low-carbon-niobium (Nb)-microalloyed pressure vessel high-strength steel materials have been developed for improved performance to fulfill this demand. Through the application of Nb-bearing plate steels, grain refinement results in a more homogeneous microstructure, improved toughness at low temperature, excellent fatigue resistance and fracture toughness, and excellent weldability result. These enhancements provide the engineering community with the opportunity to further improve their pressure vessel design and performance.