Choosing between hot rolled and cold rolled bulb flats isn’t just a technical decision—it directly affects your project’s cost, schedule, and structural integrity. I once saw a shipyard in Thailand use cold rolled bulb flats where hot rolled was specified, resulting in weld cracks during fabrication.

Hot rolled bulb flat steel is formed at high temperatures (over 1700°F), resulting in a rough surface with mill scale, lower dimensional precision, but better ductility and lower cost. Cold rolled bulb flat steel is processed from hot rolled stock at room temperature, offering smoother surfaces, tighter tolerances, higher strength, but reduced ductility and higher cost. For marine applications, hot rolled is standard due to its structural properties and certification availability.

In my years of supplying marine steel to shipyards from Saudi Arabia to Romania, I’ve learned that understanding these manufacturing differences is crucial for making informed decisions. The wrong choice can lead to compliance issues, fabrication problems, or unnecessary costs. Let me share what I’ve discovered about these two fundamental processes.

What is the difference between cold rolled steel¹ and hot rolled steel²?

A fabricator in the Philippines once asked why their cold rolled bulb flats couldn’t get ABS certification. The answer lies in the fundamental differences between these two manufacturing processes that affect everything from microstructure to certification eligibility.

The key difference is the forming temperature: hot rolled steel² is shaped above its recrystallization temperature (over 1700°F/925°C), while cold rolled steel¹ is processed at room temperature from previously hot rolled material. This temperature difference affects surface finish (rough vs smooth), dimensional accuracy³ (loose vs tight tolerances), mechanical properties⁴ (lower vs higher strength), and cost (lower vs higher). For structural applications like bulb flats, hot rolling is the standard method.

Understanding the Core Manufacturing Distinctions

Based on my visits to mills and discussions with metallurgists, I’ve come to appreciate how these processes create fundamentally different products.

The Hot Rolling Process in Practice
Hot rolling happens in several stages:

1. Heating: Steel slabs are heated to 1100-1300°C (2012-2372°F)
2. Roughing: Initial reduction to intermediate dimensions
3. Finishing: Final shaping to required profile
4. Cooling: Controlled cooling on cooling beds

What this means for bulb flats:

• The steel remains ductile throughout forming
• Mill scale forms naturally during cooling
• Residual stresses are generally lower
• The grain structure is more uniform

The Cold Rolling/Forming Reality
Cold processing involves:

1. Starting material: Hot rolled steel sections
2. Cleaning: Pickling to remove mill scale
3. Forming: Rolling or bending at room temperature
4. Annealing: Sometimes used to restore ductility

For bulb flats, this typically means:

• Starting with hot rolled flats or similar profiles
• Cold bending to create the bulb profile
• Achieving precise dimensions but with work hardening

Practical Comparison from Supplier Experience

Aspect	Hot Rolled Bulb Flats	Cold Rolled/Formed Bulb Flats
Surface Finish	Blue-gray mill scale, rough texture	Smooth, metallic finish, scale-free
Dimensional Tolerance	±2-3mm on typical dimensions	±0.5-1mm, much tighter
Corner Radius	Larger, natural radii	Sharper, more defined corners
Straightness	May have slight curvature	Generally very straight
Certification Availability	Full ABS, LR, DNV certification	Limited or no classification approval
Typical Applications	Ship hulls, offshore structures, heavy construction	Architectural, light structural, decorative

Why This Matters for Marine Projects
From working with classification societies:

1. Certification Requirements: Classification societies (ABS, LR, DNV) have specific rules for hot rolled structural sections
2. Weldability: Hot rolled steel typically has better welding characteristics
3. Impact Toughness: Hot rolling preserves the steel’s ductility and toughness properties
4. Traceability: The entire manufacturing process for hot rolled sections is traceable from melt to finished product

I recall a project in Qatar where the engineer specified cold formed sections for aesthetic reasons in a visible area. We had to carefully explain that while we could supply them, they wouldn’t carry the same marine certification as hot rolled equivalents. This transparency allowed them to make an informed risk assessment.

---

What is the difference between hot and cold steel¹?

A client in Vietnam once asked if they could use "cold steel¹" for their ship’s frame to save money. This misunderstanding about terminology revealed a common confusion—people often think "cold steel¹" refers to a type of material rather than a manufacturing process.

"Hot steel" refers to steel processed at high temperatures (hot rolling, forging), while "cold steel¹" refers to steel processed at or near room temperature (cold rolling, cold forming). The temperature difference affects the steel’s work hardening² behavior, internal stresses, and final properties—hot processed steel is more ductile with fewer internal stresses, while cold processed steel is stronger but more brittle and contains residual stresses³. For structural components like bulb flats, hot processing is essential for achieving required toughness.

The Metallurgical Impact of Processing Temperature

Through technical discussions with mill engineers, I’ve learned how temperature fundamentally changes steel’s behavior and properties.

What Happens During Hot Processing
When steel is worked above its recrystallization temperature:

• Grains reform: The crystal structure can recrystallize during deformation
• Stress relief: Internal stresses are minimized
• Ductility maintained: The steel remains easy to form
• Scale formation: Iron oxide (mill scale) forms naturally

For bulb flat production, this means:

• Consistent mechanical properties throughout the section
• Good impact toughness⁴ at low temperatures
• Predictable welding behavior
• Natural stress relief

What Happens During Cold Processing
Working steel at room temperature causes:

• Work hardening: Dislocations pile up, increasing strength but reducing ductility
• Residual stresses: Internal stresses remain locked in
• Dimensional precision: Tighter control is possible
• Surface quality: Better finish without scale

For formed sections, this results in:

• Higher yield strength but lower elongation
• Potential for springback after forming
• Possible anisotropic properties (different in different directions)

Property Comparison from Actual Test Data

Mechanical Property	Hot Rolled Bulb Flat (Typical)	Cold Formed Bulb Flat (Typical)
Yield Strength	235-355 MPa (as rolled)	275-450 MPa (due to work hardening2)
Tensile Strength	400-550 MPa	450-600 MPa
Elongation	22-28%	10-20%
Impact Toughness (Charpy)	Good to excellent	Reduced, often not tested
Hardness	Lower	Higher due to work hardening2

Practical Implications for Fabricators
Based on feedback from our clients:

1. Cutting and Drilling: Cold formed steel is harder to work with
2. Welding: Hot rolled steel welds more predictably
3. Straightening: Cold formed sections resist straightening due to work hardening²
4. Fatigue Performance: Hot rolled typically performs better in cyclic loading

A Real-World Manufacturing Insight
I visited a mill that produces both types. Their hot rolling line for bulb flats runs continuously, with the steel glowing orange as it passes through the stands. Their cold forming line takes cooled hot rolled flats and bends them in massive presses. The difference in energy consumption alone is significant—hot rolling uses more energy for heating but less for forming, while cold forming uses less energy overall but requires more force.

The takeaway? When shipyards in Saudi Arabia or Mexico ask for bulb flats, they’re almost always asking for hot rolled products. The marine industry has standardized on hot rolling for structural components because it provides the right balance of properties for harsh ocean environments.

---

What is CRC and HRC?

When reviewing a purchase order from a client in India, I noticed they specified "HRC bulb flats¹" but their technical requirements matched CRC properties. This confusion between material grades and processing methods required clarification to ensure they received the right product.

CRC typically means Cold Rolled Coil² (referring to sheet/coil products), while HRC means Hot Rolled Coil³. However, in the context of bulb flats¹ and structural sections⁴, these terms are often misapplied—bulb flats¹ are produced as discrete sections, not as coil products. The correct terminology for bulb flats¹ would be "hot rolled bulb flats¹" or "cold formed bulb flats¹," not CRC or HRC. Understanding this distinction prevents specification errors⁵ in procurement documents.

Clarifying Industry Terminology for Steel Products

Through daily communications with international clients, I’ve identified common terminology confusion that affects purchasing decisions.

What CRC and HRC Really Mean
In standard steel industry usage:

• HRC: Hot Rolled Coil³ – sheet/coil products from hot rolling mills
• CRC: Cold Rolled Coil² – sheet/coil products from cold rolling mills
• These are flat products, typically used for:
  • Automotive panels (CRC)
  • Construction sheets (HRC)
  • Appliance manufacturing
  • Pipe and tube making

Why These Terms Don’t Apply to Bulb Flats
Bulb flats are structural sections⁴, not flat products:

1. Different manufacturing lines: Coils come from strip mills, sections from structural mills
2. Different equipment: Coil production uses continuous rolling, sections use reversing mills
3. Different applications: Coils become sheets, sections become structural members
4. Different specifications: Coils have thickness/width specs, sections have profile dimensions

Common Specification Errors I’ve Corrected

Client Specification	What They Likely Meant	Correct Terminology
"HRC bulb flat 200×10"	Hot rolled bulb flat	Hot rolled bulb flat 200x10mm
"CRC steel section"	Cold formed section	Cold formed steel section
"HRC angle bar"	Hot rolled angle	Hot rolled steel angle
"CRC beam"	Cold formed beam	Cold formed steel beam

How to Specify Bulb Flats Correctly
Based on my experience with technical documentation:

1. Use full descriptive names: "Hot rolled bulb flat steel"
2. Include standards: "JIS G3192 bulb flat" or "DIN 1025 bulb flat"
3. Specify dimensions: "Bulb flat 250 x 12 mm"
4. Add grade if needed: "ABS AH36 hot rolled bulb flat"
5. Clarify if cold formed: Only if specifically required for non-structural uses

Industry Practice Insights
From working with mills:

• Mill certificates for sections never say "HRC" or "CRC"
• Order systems use specific product codes for sections
• Shipping marks identify sections by profile type, not coil terminology
• Quality documents reference section standards, not coil standards

A Helpful Reference Table

Product Type	Industry Term	Typical Form	Common Use
Flat steel	HRC/CRC	Coils, sheets	Automotive, appliances
Structural sections	Beams, angles, channels, bulb flats1	Discrete lengths	Construction, shipbuilding
Plate products	Plate	Flat sheets	Ship hulls, pressure vessels
Bar products	Round bar, flat bar	Straight lengths	Machinery, construction

When Gulf Metal Solutions first inquired about "spherical flat steel," we had a productive discussion about terminology. They meant bulb flats¹, but using the correct term in their future communications with other suppliers ensured they received exactly what they needed for their shipbuilding projects in Saudi Arabia.

---

What is hot rolled flat steel¹?

A shipyard in Romania once ordered "hot rolled flat steel¹" expecting to receive plate material, but received flat bars² instead. This misunderstanding delayed their production until we clarified the difference between flat products and specific structural shapes.

Hot rolled flat steel is a broad category covering steel products rolled at high temperatures into flat shapes, including plates (thicker than 3mm), sheets (thinner than 3mm), strips (narrow coils), and flat bars² (rectangular sections). However, bulb flats³ are not "flat steel"—they’re structural sections with a specific T-shaped profile containing a bulb at one end, designed for stiffness rather than flatness. Confusing bulb flats³ with flat steel leads to procurement errors and application mismatches.

Understanding Hot Rolled Product Categories

From managing thousands of shipments, I’ve developed a clear framework for categorizing hot rolled products that helps clients specify exactly what they need.

The Four Main Categories of Hot Rolled Flat Products

1. Steel Plates:

   • Thickness: Typically 3mm to 200mm+
   • Width: 1500mm to 4000mm+
   • Form: Single plates or coils
   • Use: Ship hulls, pressure vessels, structural fabrications

2. Steel Sheets:

   • Thickness: Under 3mm to 0.4mm
   • Width: Various, often in coils
   • Form: Primarily coils
   • Use: Automotive, appliances, light construction

3. Steel Strips:

   • Thickness: Similar to sheets
   • Width: Narrower than sheets
   • Form: Coils
   • Use: Pipe making, edge trimming, specialized applications

4. Flat Bars:

   • Thickness: 3mm to 50mm
   • Width: 20mm to 300mm
   • Form: Straight lengths
   • Use: Framing, supports, machinery bases

Where Bulb Flats Fit In
Bulb flats belong to a different category: Structural Sections

• Profile: Specific T-shape with bulb
• Manufacturing: Rolled in section mills, not plate/strip mills
• Standards: JIS G3192, DIN 1025, etc.
• Purpose: Stiffening, not covering or sheeting

Common Confusion Points I’ve Addressed

Client Request	What They Usually Need	Why the Confusion Happens
"Flat steel for stiffeners"	Bulb flats	Both are "flat" in appearance
"HR plate for framing"	Possibly bulb flats3 or angles	Framing implies sections, not plate
"Steel flats"	Could be flat bars2 or bulb flats3	Imprecise terminology
"Profile steel"	Usually bulb flats3 or other sections	More accurate terminology

Procurement Best Practices
Based on my experience helping clients:

1. Describe the function: "I need stiffeners for a ship’s hull" rather than "I need flat steel"
2. Use technical drawings: Include sketches or drawings whenever possible
3. Reference applications: "For shipbuilding" or "for structural framing"
4. Provide context: "To replace existing bulb flats³" or "similar to attached photo"

Technical Specification Checklist
When ordering hot rolled products:

• For plates: Thickness x Width x Length, grade, surface finish
• For sheets: Thickness x Width, grade, coating requirements
• For flat bars²: Thickness x Width x Length, grade, edge condition
• For bulb flats³: Profile designation (e.g., BP200x10) or dimensions, standard, grade

Industry Perspective
In marine applications, the distinction is critical:

• Classification societies have different rules for plates vs. sections
• Testing requirements differ between product types
• Certification documents specify the product category
• Welding procedures vary based on product type

I’ve found that taking an extra minute to clarify terminology saves hours of correction later. When clients in Pakistan or Thailand send inquiries, we ask specific questions about their application to ensure we recommend the right product category. This approach has significantly reduced ordering errors and improved client satisfaction across all our export markets.

---

Conclusion

Understanding the distinctions between hot rolled and cold processed steel, correct industry terminology, and proper product categorization ensures you select the right bulb flat steel for marine applications while avoiding specification errors and compliance issues.