An enquiry that reads “Q235 or equivalent” looks harmless until the mill certificate arrives. The steel is quoted as SS400, the fabricator’s engineer asks why the document says 400 when the drawing says 235, and the order stalls while two people argue about a number neither of them chose. Nothing is wrong with either figure. They are measurements of different properties, in different units, under different standards — and “equivalent” conceals that.

The number is not always the same property

Chinese GB grades are named for minimum yield strength in MPa. Q195 guarantees 195 MPa, Q215 guarantees 215 MPa, Q235 guarantees 235 MPa and Q345 guarantees 345 MPa. The European S-series works the same way, which is why one H-beam range can list Q235, Q345, ST37-2, ST52, Q420, Q460, S235JR, S275JR and S355JR in a single line.

JIS does not. SS400 is a tensile designation: 400 MPa minimum tensile strength, with a yield point near the 235 family — the supplier’s own page equates it with Chinese Q235B. ASTM is neither: A36 means 36 ksi minimum yield, roughly 250 MPa. In API 5L the X-number is the yield strength in ksi: X42 is 42 ksi, X70 is 70 ksi.

So 235 is MPa yield, 400 is MPa tensile, 36 is ksi yield and X42 is ksi yield. Four numbers on one order, three different meanings.

Why “equivalent” survives the quote but not the design

Equivalence tables normally compare yield and tensile only. A design may lean on properties that never appear. Yield steps down as the section thickens, so a 235 MPa guarantee on 16 mm plate does not carry over to 40 mm. EN grades carry a toughness suffix — JR, J0, J2, K2 — that sets the Charpy test temperature, and GB grades make the same distinction through the A/B/C/D suffix. Delivery condition matters too: as-rolled, normalized and thermomechanically rolled weld differently.

For a statically loaded base plate or a fence post, none of this decides anything. For a welded node in a cold climate, or anything holding pressure, it decides everything. The chemistry tables show where the money is: ASTM A53 caps copper, nickel and chromium at 0.40 per cent and molybdenum at 0.15 per cent, and those residuals drive carbon equivalent, preheat and procedure qualification.

Carbon plate and coil: four naming logics on one stock list

Walk a hot rolled warehouse and the same rack carries grades named four different ways. The Q series is yield-coded. A36 is ksi-coded. ST37 and ST37-2 are legacy DIN names from the old kg/mm² convention. SPHC carries no number at all — it is a JIS commercial-quality class for hot rolled plate, not a strength class, which means ordering “SPHC equivalent to Q235” is asking for a comparison the designation does not support.

Then there are composition shorthands: 45# means roughly 0.45 per cent carbon, while 16Mn and Q345 sit in the manganese high-strength group. A mill holding all nine is not offering nine interchangeable products. It is offering nine different promises, and the only way to know which one you are buying is to say which standard the certificate is written against.

Sections: the same beam sold under three standards

Shape and grade are independent, and confusing them is the most common error in section orders. IPE 450 is a European designation decoded as follows: I for I-section, P for parallel flanges, E for the S270 quality class under EN 10025-2:2004, 450 for the nominal height in millimetres. The supplier’s own page equates that class with Q235B in GB/T 700. Ordering “IPE450” therefore fixes the geometry and says nothing about the steel.

The same catalogue lists H-beams as A36/A992, Q235B/Q345B and ASTM A29M product, with flanges from 50 to 400 mm, webs from 100 to 900 mm, thicknesses of 6 to 35 mm, lengths of 6 to 12 m, tolerance of plus or minus 1 per cent, theoretical-weight invoicing and a 25-tonne minimum. Those commercial terms are identical across standards. The mechanical guarantee is not.

Pipe: the X-number is the yield, and PSL1 is not PSL2

Pipe grades are the easiest to misread because they look like a ladder. ASTM A53 — also issued as ASME SA53 — gives Grade A 48,000 psi tensile and 30,000 psi yield, Grade B 60,000 and 35,000, in seamless, ERW and furnace-weld types from half an inch to 36 inches nominal bore. A106 covers high-temperature service at 48,000, 60,000 and 70,000 psi for Grades A, B and C. Every length gets a hydrostatic test or a full-body nondestructive electric test.

API 5L is different again: the grade number is the yield strength, and the PSL level is the real specification. PSL1 runs X42 at 42 ksi yield and 60 ksi tensile through X70 at 70 and 82 — in metric terms, at least 290 MPa yield at X42 and at least 483 MPa at X70. PSL2 keeps the same X-number but tightens phosphorus from 0.030 to 0.025 per cent and sulphur from 0.030 to 0.015 per cent, and adds toughness and nondestructive testing obligations that PSL1 does not carry. An order that names the grade but not the PSL is only half written.

Stainless and coated sheet: one alloy, several names

Stainless is the cleanest case, because the alternative designations are published. Type 304 is 1.4301 in the European Werkstoff system and SUS304 in JIS; 304L is 1.4307. 316 is 1.4401 and 316L is 1.4404. The mechanical differences still matter — 304 at 600 MPa tensile and 210 MPa yield with 60 per cent elongation, against 304L at 530 and 200 with 50 per cent — and dual-certified plate exists because the two specs overlap. Published tables are typically quoted for flat rolled products under ASTM A240/A240M; bar and pipe values differ.

Coated sheet is where naming gets crowded. One galvanized product legitimately appears as SGCC under JIS G3302, DX51D+Z under EN, CS Type B under ASTM A653M and ST02Z under DIN, and a single product page carries all three standard bodies. Hardness is quoted on its own scale — soft at HRB 60, medium at 60 to 85, full hard at 85 to 95 — so a coated sheet order needs grade, coating mass and temper stated separately.

What to write on the enquiry instead

Name the governing standard first, then the grade: EN 10025-2 S355JR, or ASTM A36, or GB/T 700 Q235B. Not “Q345 or equivalent”. Then name the property that actually governs the design — yield at the thickest section ordered, the Charpy temperature if the structure works below freezing, carbon equivalent if it will be welded, coating mass if it will be exposed.

State which certificate you want and who reads it. Mill test certificates to EN 10204 3.1B are standard on pressure and line pipe; plate and coil shipments should carry a heat number tying each bundle back to a test result. When a substitution is offered, ask for the measured values rather than the substitute grade name. Hot rolled, pipe and stainless ranges are all stocked in several standards at once, so “can you supply it” is rarely the useful question. “Which standard will the certificate be written to” is.