F460 Heavy Steel Plates for Offshore Structure and Shipbuilding Produced by Thermomechanical Control Process

Abstract

Key parameters for controlled rolling and accelerated cooling process have been determined to industrially produce 45–50 mm heavy plates of microalloyed low carbon-equivalent SiMnCrNiCu steel for offshore structures and shipbuilding. The plates were hot rolled from continuously cast slabs with a four-high 5000 mm width mill and cooled with an accelerate-cooling system. The process was characterized by heavy finish rolling reduction ratio over 63% in austenite non-recrystallization region and interrupted accelerated cooling at 460°C to form quasipolygonal ferrite and acicular ferrite as majorities of microstructure. Yield-strength greater than 460 MPa was achieved at room temperature, and Charpy V notch impact energies of 150 to ~300 J were secured even at –80°C. Coarse granular bainite and/or degenerate upper bainite were identified harmful, causing cleavage fracture. The correlation between the fraction of high-angle grain boundaries (≥15 deg) and ductile-brittle transition temperature was derived quantitatively for the advanced heavy plates. The effect of heat input during heavy plate welding on the microstructure and mechanical properties in the coarse-grained heat affect zone was discussed.