Steel Plate Grades Explained

When a buyer sends us a drawing marked "S355JR" and asks for A572-50 instead — or the reverse — the answer is usually yes, but not always. Understanding what the grade designation actually guarantees makes you a sharper buyer and prevents expensive surprises at third-party inspection.

What the Grade Names Actually Mean

Each standard system encodes yield strength, quality and sometimes temperature class:

• ASTM A36 — the legacy US mild structural plate: 250 MPa min yield (plates ≤ 200 mm), no impact requirement.
• ASTM A572 Gr.50 — high-strength low-alloy (HSLA): 345 MPa min yield (≤ 65 mm), columbium-vanadium microalloyed.
• EN S235JR / S355JR +J2 — European: S = structural, number = yield in MPa, JR/J2/K2 = impact test class (20°C / −20°C / −40°C... actually +20/−20/−30°C).
• GB Q235B / Q355B — Chinese: Q = 屈服 (yield), number = yield, letter = impact class (A none, B +20°C, C 0°C, D −20°C, E −40°C).
• AH36/DH36/EH36 — classification society hull grades: 355 MPa yield; letters = impact test at 0/−20/−40°C, society-stamped certificates mandatory.

The Comparison Table

Grade	Standard	Min yield (≤16 mm)	Tensile	Impact test	Relative price*
A36	ASTM	250 MPa	400–550 MPa	None	100
Q235B	GB/T	235 MPa	370–500 MPa	+20°C, 27J	98–102
S235JR	EN	235 MPa	360–510 MPa	+20°C, 27J	105–110
A572-50	ASTM	345 MPa	≥ 450 MPa	None	110–118
S355JR	EN	355 MPa	470–630 MPa	+20°C, 27J	112–120
Q355B	GB/T	355 MPa	470–630 MPa	+20°C, 34J	110–118
AH36	Class rules	355 MPa	490–630 MPa	0°C, 34J	125–140

*Relative to A36 = 100; moves with mills and alloy surcharges.

The Substitution Table Buyers Ask For

Specified	Common substitute	Caution
A36	Q235B / S235JR	A36's 250 vs 235 MPa yield — most fabricators accept; confirm with engineer on governed jobs
A572-50	Q355B / S355JR	Direct equivalent; check CEV ≤ 0.45 for site welding
S355J2	Q355D	Both −20°C / −40°C impact classes match
AH36	—	No substitute: must be class-society certified, stamp intact

Weldability: The CEV Check

Carbon equivalent (CEV, IIW formula) predicts hardenability and preheat need. Typical CEVs: A36 ≈ 0.40 max, Q235B ≤ 0.38, Q355B ≤ 0.45, A572-50 ≤ 0.48. All of these weld with standard low-hydrogen electrodes without preheat below ~25–40 mm thickness in mild conditions. When a supplier's MTC shows CEV above the spec limit, reject the plate — no exceptions on structural work.

Five Buying Rules

1. Match the design code, not just the number. A project to AWS D1.1 assumes ASTM plate; EN 1090 assumes EN. Dual-certified plate (e.g. S355JR+AR to also meet Q355B) solves mixed-code projects.
2. Insist on EN 10204 3.1 MTC with every consignment — type 3.2 with third-party countersignature for class or critical work. See our MTC guide.
3. Check impact class when cold matters. Q235B (B class) is tested at +20°C; for outdoor structures in cold climates step up to C/D class or S355J2.
4. Watch thickness effect on yield. A572-50 drops to 290 MPa min yield above 65 mm; Q355 similar. Thick plates may need TMCP route to hold grade.
5. Verify plate marks on arrival. Stencil/brand with heat number matching the MTC — that is what your inspector checks first. See our inspection checklist.