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 highangle 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.

Micromechanism of Decrease of Impact Toughness in Coarse-Grain Heat-Affected Zone of HSLA Steel with Increasing Welding Heat Input

“…Furthermore, the toughness in the CGHAZ decreased with the increasing welding heat input. [16][17][18] At high heat input, the grain size in CGHAZ could reach more than ten times of the original average values.…”

“…Therefore, more studies were concentrated on studying microstructures and properties of the heat-affected zone (HAZ) by varying peak temperatures, heat inputs, and alloy compositions. [7][8][9][10][11][12][13][14][15][16][17][18][19] In Reference 12 coarse austenite grain size associated with coarse M-A constituent along grain boundary was revealed as the dominant factor in promoting brittle fracture. The combination of fine prior austenite grain size and smaller M-A constituent was favorable in obtaining high toughness.…”

Microstructural Characterization and Mechanical Properties across Thickness of Ultra‐Heavy Steel Plate

“…However, in most cases, low‐carbon low‐alloyed steels are preferred for heavy steel plate considering weldability and alloying cost. To achieve the best combination of strength and toughness for low‐alloyed steels, microstructures like lath martensite (LM), lath bainite (LB), acicular ferrite (AF), and quasi‐polygonal ferrite (QAF) with fine effective grain size are usually desired, while coarse granular bainite (GB) and coarse martensite/austenite (M/A) constituent deteriorate the low‐temperature toughness …”

Effect of Ta and Ti on Modified Reduced Activation Ferritic/Martensitic Steels with a Thermo‐mechanical Control Process

“…Finally, the grain refinement by TMCP was also an established strategy for the comprehensive improvement of mechanical properties. [ 27,28 ] In summary, after the overall modification of the microstructure, M‐RAFM–TaTi steels exhibited better properties than traditional RAFM steels.…”