20th International Forgemasters Meeting - IFM 2017

Offshore structure is the large installation to facilitate drilling, refining, saving, and producing oil and gas. Hence, offshore structure should maintain long term safety of structure against severe external environments such as storm waves and low temperature. Therefore, heavy thick steel plates required with high strength and toughness in low temperature have been needed as the engineering applicable materials for offshore structure. Especially, offshore structures are being changed toward to the trend of large size by multipurpose in recent years so that high strength of structures is getting important for weight reduction. The purpose of this paper is to report the development of heavy thick steel plate with an high strength and high toughness at low temperature. In more detail, the product is welded element for jack up offshore plant, composing of the rack & the chord plates. It is required to high technology in all processes including steel making, forging, forming, heat treatment and welding. Optimum manufacturing condition including forming, flame cutting and welding was investigated to develop the heavy thick steel plate with high strength and excellent toughness in low temperature for offshore plants. After making ingot, it was subjected to manufacture a slab type shape with open die forging and then, control a final plate shape with rolling. The rack plate of 210mm thickness and chord plate of 120mm thickness was produced. Weldabilty tests were also performed about rack and chord materials. After heat treatment, the rack plate was conducted by flame cutting and the chord plate was deformed by hot or cold bending and then, both products of the flame cut rack plate and the deformed chord plate were welded each other using approved Welding Procedure Specification(WPS) by Submerged Arc Welding(SAW). The plates were tested mechanical properties including tensile, impact properties, and high cycle fatigue, etc, and the welded element was subject to test non-destructive inspection. Classification class qualified the properties of the plates and the welded element, and approved all manufacturing process.