1. Q: What is the basic definition of GB/T 712 DH790 steel pipe?
A: It is an ultra-high-strength, low-temperature toughness steel specified in the Chinese standard for shipbuilding structural steel (GB/T 712). In DH790:
"D" stands for the -40°C low-temperature toughness grade (stricter than the -20°C rating of AH790);
"H790" indicates a yield strength of ≥790 MPa. It is currently the highest-strength steel used in shipbuilding and offshore engineering, designed specifically for extreme environments such as polar vessels and deep-sea equipment.
2. Q: Compared with the same series of AH790 and EH690, what are the core advantages of DH790?
A:
Strength and Toughness Balance: Maintaining an ultra-high strength of 790 MPa, it maintains an impact energy of ≥47J at -40°C (AH790 only requires -20°C);
Adaptability to Extreme Environments: Compared to EH690 (which has lower strength at -60°C), it is more suitable for applications requiring a balance between strength and low-temperature performance, such as LNG carriers on Arctic routes;
Welding Process Optimization: Through Ni-Cr-Mo-V composite microalloying, the carbon equivalent (Ceq) is controlled below 0.45%, reducing the difficulty of welding high-strength steel.
3. Q: What are the chemical composition and key mechanical properties of DH790?
A:
Composition Design:
Low carbon (C ≤ 0.10%), high nickel (Ni 2.5-3.5%), molybdenum (Mo 0.3-0.6%), and extremely pure sulfur and phosphorus (S/P ≤ 0.005%);
Addition of V and Ti refines the grain size and improves Z-axis lamellar tearing resistance. Mechanical Specifications:
Yield Strength ≥ 790 MPa, Tensile Strength ≥ 890-1040 MPa;
Elongation ≥ 14%, -40°C Impact Energy ≥ 47 J, Z-axis Reduction of Area ≥ 35%.
4. Q: Why is DH790 called the "core material for polar equipment"?
A:
Key components of polar icebreakers, such as the bow of Russia's Arktika-class icebreakers, must simultaneously withstand ice impact (high strength) and -50°C temperatures (to prevent brittle fracture).
Deep-sea pressure-resistant structures: Used in manned submersible pressure chambers (such as the upgraded material for the Struggler), capable of withstanding external pressures up to 1000 MPa.
Military applications: Nuclear submarine pressure hulls can reduce weight by over 20% while meeting stealth requirements (low magnetism).
5. Q: What are the technical difficulties in producing and using DH790?
A:
Metallurgy: A triple process of electric furnace + LF refining + vacuum degassing is required to ensure molten steel purity (O ≤ 20 ppm).
Heat Treatment Control: Quenching and tempering (quenching and tempering) is required, with the tempering temperature accurate to within ±10°C (typically 580-620°C).
Welding Technology:
Specialized welding consumables: such as AWS A5.28 ER120S-G (matching 790 MPa strength).
Process Parameters: Preheat 150-180°C, interpass temperature ≤ 200°C, and dehydrogenation at 300°C for 2 hours immediately after welding.
Certification Requirements: Requires passing CTOD (crack tip opening displacement) and detonation tests by classification societies such as DNV GL and CCS.
