Bulb Flat Steel is a high-efficiency structural section specifically designed for shipbuilding and offshore engineering. Its cross-section features a flat bar with a rounded bulb (semi-circular head) at one end, resembling a "T" or "L" shape, offering excellent bending and torsional resistance with high structural efficiency. Due to its lightweight, high strength, and superior weldability, bulb flats are widely used in hull structures, deck frames, bulkhead stiffeners, platform walkways, bridges, and heavy equipment supports. We supply both hot-rolled and welded bulb flats, manufactured strictly to international classification society standards, with high dimensional accuracy, excellent low-temperature toughness, and weldability-making them a key structural material in modern shipbuilding and offshore projects.
Key Features
Standards:
Chinese Standards:
GB/T 704-2016 (Hot-Rolled Flat Steel)
CB/T 3353-1994 (Marine Bulb Flats)
GB/T 712-2022 (Structural Steels for Ships and Offshore Units)
International Standards:
JIS G 3192 (Japanese Bulb Flat series, e.g., BF65×12, BF100×16)
BS 100-1 (British Standard for Structural Steel)
ASTM A131/A131M (Standard for Structural Steel for Ships)
ISO 630-2 (Structural Steels – Part 2: Plates and Wide Flats for Shipbuilding)
Classification Society Rules:
CCS "Rules for Classification and Construction of Sea-Going Steel Ships"
ABS "Rules for Building and Classing Steel Vessels"
DNV-RU-SHIP Pt.3 Ch.2 (Hull Structures)
Materials:
General Strength Grades: A, B (for ambient temperature applications)
High-Strength Grades:
AH32, DH32, EH32 (Yield ≥315MPa)
AH36, DH36, EH36 (Yield ≥355MPa)
AH40, DH40, EH40 (Yield ≥390MPa)
(A=0°C, D=-20°C, E=-40°C impact testing)
Corrosion-Resistant Grades: Seawater corrosion-resistant steels per GB/T 712 or class rules
Stainless Steel Bulb Flats (Special Applications): 316L, 2205 Duplex (for chemical tankers, luxury yachts)
Dimensions:
Height (H): 50mm – 180mm
Web Thickness (t1): 6mm – 16mm
Bulb Diameter (D): Slightly larger than web thickness
Flange Width (B): 30mm – 80mm
Common Sizes:
BF50×6, BF65×8, BF80×10, BF100×12, BF125×14, BF150×16, BF180×18
(BF = Bulb Flat, e.g., BF100×12: 100mm height, 12mm web thickness)
Length: 6m, 9m, 12m standard; up to 18m available
Custom Fabrication: Pre-cutting, beveling, curving, drilling available
Applications:
Shipbuilding: Hull frames, longitudinal stiffeners, deck girders, bulkhead stiffeners
Offshore Engineering: Platform walkways, ladders, railings, module supports
Bridges: Pedestrian bridge girders, maintenance access frames
Heavy Machinery: Equipment bases, crane outriggers
Construction: Special-shaped beams, reinforcement members
Military & Special Vessels: Warships, icebreakers, research vessels
Certifications & Approvals:
Classification Society Certifications: CCS, ABS, DNV, LR, BV, NK, KR, RINA
Provide: Type Approval, MTC 3.1/3.2, Factory Survey Reports, CoC
Owner & Engineering Company Approvals:
ISO 9001, ISO 14001, ISO 45001
PED 2014/68/EU (for offshore pressure-bearing structures)
NORSOK M-650 / M-601
Approved by CSSC, COSCO Shipping, Jiangnan Shipyard, Hudong-Zhonghua, HD Hyundai Mipo, Mitsubishi Shipbuilding, Meyer Werft, etc.
Common Customer Questions (Q&A Format)
Q: What is bulb flat steel? How is it different from regular flat bar?
A: Bulb flat has a rounded bulb for enhanced bending/torsional strength; regular flat bar is rectangular, used for light-duty or connecting parts.
Q: What is the difference between AH36 and DH36?
A: Both have yield strength ≥355MPa, but impact testing temperature differs: AH36 at 0°C, DH36 at -20°C, suitable for colder environments.
Q: Do you offer custom sizes?
A: Yes, we can customize height, thickness, length, and fabrication (bending, holes) per drawings or class rules.
Q: Do you provide corrosion protection?
A: Yes, options include hot-dip galvanizing, shop primer (e.g., inorganic zinc), epoxy coating to meet shipyard requirements.
Q: How is weldability?
A: AH/DH/EH grades have excellent weldability using GMAW, SMAW, etc. Preheating (100–150°C) required for thick sections.
Q: Do you provide Material Test Certificates (MTC)?
A: Yes, class-approved MTC 3.1/3.2 provided per batch with heat number, chemistry, tensile/impact data, and compliance.
Q: What is the maximum single length?
A: Standard 12m, up to 18m; longer pieces can be segmented for site assembly.
Q: Is it suitable for polar vessels?
A: Yes, EH40 grade with -40°C impact toughness meets Arctic vessel structural requirements.
Q: How is dimensional accuracy ensured?
A: Specialized rolling mills and CNC straightening ensure key dimensions (height, thickness) within ±1.5mm, uniform bulb shape.
Q: What is the lead time?
A: Standard sizes: 15–30 days (including class survey); custom orders: 30–60 days depending on complexity.
Q: Can you provide BIM models or cutting plans?
A: Yes, we offer 3D models, NC cutting files, and nesting optimization to support digital shipbuilding.
Q: Can it be used in stainless steel hulls?
A: Yes, we supply 316L or 2205 duplex stainless steel bulb flats for yachts, chemical tankers, and corrosive environments.
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