A steel for grade r6 offshore mooring chain with high strength and high toughness and its chain use in anchoring and mooring floating bodies with cathodic protection

Abstract

The present application relates to a steel for grade R6 offshore mooring chain for use in anchoring and mooring floating bodies with cathodic protection: the chemical composition are C 0.18˜0.24%, N 0.006˜0.024, P 0.005˜0.025, S≤0.005, Si 0.15˜0.35, Mn 0.20˜0.40, Cr 1.40˜2.60, Ni 0.80˜3.20, Mo 0.35˜0.75, Cu≤0.50, Al≤0.02, Ti≤0.005, V 0.04˜0.12, Nb 0.02˜0.05, Ca 0.0005˜0.004, O≤0.0015, H≤0.00015, the balance is Fe: the total content of alloy ΣM=(Si+Mn+Cr+Ni+Mo+Cu), 3.4<ΣM≤6.8; the total content of microalloy ΣMM=(Ti+Al+Nb+V), 0.065≤ΣMM≤0.194. The corrosion potential is adjusted to prevent hydrogen embrittlement caused by cathodic overprotection on the basic premise of maintaining the strength, toughness and low corrosion rate of the steel. Where V is only used for strengthening, and the content of N in VCN is increased, especially for the increase of the temperature for chain quenching to make M3C, M2C and VCN fully dissolved in solid solution and fully precipitated in tempering, which improves the precipitation strengthening effect.

Description

TECHNICAL FIELD[0001]The present application relates to alloy steel and ferrous metallurgical products for offshore engineering, in particular a grade R6 steel and chain for offshore mooring in the steel series for offshore mooring chain and the evaluation of its offshore environmental performance degradation resistance.BACKGROUND ART[0002]A mooring system is acquired for positioning and mooring with floating bodies in the process of the marine exploration, deep-sea oil and gas extinction, national defense construction and so on. The main component of the mooring system is offshore mooring chain by the steel. According to the standard in DNVGL (Det Norske Veritas-Gemtanischer Lloyd) named Offshore Standards, DNVGL-OS-E302 Offshore mooring chain. Edition July 2018, the offshore mooring chain can be classified into six grades called R3, R3S, R4, R4S, R5, and R6 according to its strength grading. The tensile strength of the chains at all grades after overall quenching-tempering shall b...

AI Technical Summary

Benefits of technology

The patent aims to improve the strength and toughness of chains while reducing corrosion. This is achieved by controlling the contents of alloy and microalloy to enhance hardenability and lower corrosion potential. The invention also adjusts the corrosion potential to prevent hydrogen embrittlement caused by cathodic overprotection while maintaining the strength, toughness, and low corrosion rate of the steel.

Problems solved by technology

There have even been catastrophic accidents in which the platform capsized caused by the fracture of the chain.

This is a new challenge in the research and development of the ultrahigh-strength chain.

The air cooling process is impossible to be implemented in the continuous heat treatment production line to avoid the tow-temperature phase transformation crack although this patent requires air cooling after heat treatment.

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