Carbon Steel Grades: A Complete Guide (A36, Q235B, Q355B, S235JR & More)

What is Carbon Steel?

Carbon steel is the most widely used type of steel, accounting for approximately 90% of global steel production. It’s an alloy of iron and carbon (with manganese and trace impurities), where carbon is the primary alloying element that determines strength and hardness.

Carbon steel is valued for its:

• Low cost and wide availability
• Excellent weldability and machinability
• Good strength and ductility
• Versatility across thousands of applications

This guide covers the most common carbon and low-alloy structural steel grades you’ll encounter in international trade.

How Carbon Steel is Classified

Carbon steels are generally classified by their carbon content:

Most structural and general-purpose steel falls into the low carbon (mild steel) category.

Most Common Structural Carbon Steel Grades

1. ASTM A36 (United States)

A36 is the standard structural carbon steel in North America. The “36” refers to its minimum yield strength of 36,000 psi (250 MPa).

• Standard: ASTM A36/A36M
• Carbon: ≤ 0.29% (≤100mm thick)
• Yield Strength: 250 MPa (36 ksi) for ≤50mm
• Tensile Strength: 400–550 MPa (58–80 ksi)
• Elongation: 20%
• Key Features: Excellent weldability, ductile, widely available
• Applications: Buildings, bridges, platework, machinery, general fabrication

2. GB Q235B (China)

Q235B is the most common structural steel in China and much of Asia. Q = yield strength, 235 = 235 MPa, B = quality grade B (impact tested at +20°C).

• Standard: GB/T 700 / GB/T 3274
• Carbon: ≤ 0.20% (≤16mm thick)
• Yield Strength: 235 MPa for ≤16mm
• Tensile Strength: 375–500 MPa
• Elongation: 26%
• Key Features: Lowest cost structural grade, excellent formability, abundant supply
• Applications: General structures, profiles, pipes, sheet metal, non-critical components

Near-equivalent to: ASTM A36, EN S235JR

3. GB Q355B (China)

Q355B (formerly Q345B) is China’s standard low-alloy high-strength structural steel. It achieves higher strength through manganese and microalloying elements (V, Nb, Ti).

• Standard: GB/T 1591-2018
• Carbon: ≤ 0.24%
• Yield Strength: 355 MPa for ≤16mm
• Tensile Strength: 470–630 MPa
• Elongation: 21%
• Key Features: 50% stronger than Q235B, good weldability, multiple quality levels (B/C/D/E)
• Applications: Heavy structures, bridges, cranes, pressure vessels, wind towers

Near-equivalent to: ASTM A572 Grade 50, EN S355JR

4. EN S235JR (Europe)

S235JR is Europe’s standard mild structural steel. S = structural, 235 = 235 MPa yield, JR = 27J impact at room temperature.

• Standard: EN 10025-2
• Carbon: ≤ 0.17–0.20% (depends on thickness)
• Yield Strength: 235 MPa for ≤16mm
• Tensile Strength: 360–510 MPa
• Elongation: 26%
• Impact: 27 J at +20°C
• Key Features: European standard, certified impact resistance, good weldability
• Applications: Building structures, machinery, general engineering

Near-equivalent to: ASTM A36, GB Q235B

5. EN S355JR (Europe)

S355JR is Europe’s standard high-strength structural steel. Available in JR/J0/J2/K2/NL quality levels for different temperature toughness.

• Standard: EN 10025-2 / -3
• Carbon: ≤ 0.24%
• Yield Strength: 355 MPa for ≤16mm
• Tensile Strength: 470–630 MPa
• Elongation: 21%
• Impact: 27 J at +20°C
• Key Features: High strength, multiple quality options, widely specified in Europe
• Applications: Heavy construction, bridges, offshore, lifting equipment

Near-equivalent to: ASTM A572 Grade 50, GB Q355B

6. ASTM A572 Grade 50 (United States)

A572 Grade 50 is a high-strength low-alloy (HSLA) structural steel widely used in North America.

• Standard: ASTM A572/A572M
• Yield Strength: 345 MPa (50 ksi)
• Tensile Strength: 450–600 MPa (65–87 ksi)
• Elongation: 18%
• Key Features: High strength-to-weight ratio, good formability
• Applications: Structural shapes, bridges, heavy equipment

Near-equivalent to: GB Q355B, EN S355JR

7. SS400 (Japan)

SS400 is Japan’s standard general-purpose structural steel (JIS G3101).

• Standard: JIS G3101
• Yield Strength: ≥ 245 MPa (for ≤16mm)
• Tensile Strength: 400–510 MPa
• Elongation: 21%+ (depending on thickness)
• Key Features: Japanese industrial standard, good general properties
• Applications: General structural use across Japanese and SE Asian markets

Near-equivalent to: ASTM A36, GB Q235B, EN S235JR

Quick Comparison Table

Low Carbon vs High Strength: Which to Choose?

Choose Mild Steel (Q235B / A36 / S235JR) If:

• ✅ Cost is the primary concern
• ✅ Loads are moderate
• ✅ Excellent formability is needed (bending, stamping, deep drawing)
• ✅ Simple welding and fabrication
• ✅ Structure is non-critical or lightly loaded
• ✅ Maximum availability and shortest lead time

Choose High-Strength Low Alloy (Q355B / A572 Gr.50 / S355JR) If:

• ✅ Need to reduce weight and material usage
• ✅ Heavy loads or long spans
• ✅ Space-constrained designs
• ✅ Engineering specifications require it
• ✅ Life-cycle cost favors less material + higher grade
• ✅ Bridges, cranes, heavy machinery

The Cost / Weight Tradeoff

High-strength steel costs more per ton, but you use less of it:

Rule of thumb: If strength is the limiting factor, high-strength steel usually saves money. If stiffness, corrosion allowance, or minimum thickness governs, mild steel is cheaper.

Pipe and Tube Grades

Carbon steel pipes have their own grade systems:

Seamless Pipes

• ASTM A106 Gr. B: Standard seamless carbon steel pipe for high-temperature service
• API 5L Grade B: Line pipe for oil and gas transmission
• GB 8163 20#: Chinese seamless steel pipe for fluid transport
• ASTM A53 Gr. B: Seamless and welded carbon steel pipe

Welded Pipes

• ASTM A53 Gr. B (ERW): Electric resistance welded pipe
• API 5L X42–X80: Higher strength line pipe grades
• GB/T 3091 Q235B/Q355B: Chinese welded steel pipe for general use

Galvanized Carbon Steel

Carbon steel is often hot-dip galvanized (HDG) for corrosion protection. Common galvanized grades:

• Q235B HDG: Most common in China — economical and widely available
• A36 HDG: Common in North America
• S235JR HDG: Common in Europe

Galvanizing typically adds 30–50% to the cost of carbon steel but provides 10–30 years of corrosion protection depending on environment.

How to Specify Carbon Steel Correctly

When ordering carbon steel, always specify:

1. Grade and standard (e.g., “Q235B per GB/T 700” or “A36 per ASTM A36”)
2. Product form (plate, sheet, coil, pipe, H-beam, angle, etc.)
3. Dimensions (thickness, width, length, diameter, wall thickness)
4. Surface condition (black, mill finish, galvanized, painted)
5. Certification (MTC to EN 10204 3.1 or 3.2 as needed)
6. Quantity (tons or pieces)

Common Carbon Steel Products

Conclusion

Carbon steel is the foundation of modern industry — versatile, affordable, and available in grades optimized for every application. Understanding the different grades and their equivalents across standards is essential for international sourcing.

Key takeaways:

• Mild steel (Q235B / A36 / S235JR) is the economical choice for general structures
• High-strength steel (Q355B / A572 Gr.50 / S355JR) saves weight and often total cost for heavy-load applications
• Most grades are near-equivalent across standards — substitution is usually possible with engineering approval
• Galvanizing is the most cost-effective way to add corrosion protection

At {{ .Site.Params.companyShortName }}, we supply all common carbon steel grades in every product form. Contact us for a competitive quote — we can help you select the right grade for your application.