Plate arriving on a truck looks like plate. Two bundles of the same size and finish can behave very differently once they reach a fabrication shop, and the difference is not visible: it is written on the mill test certificate. For hot rolled carbon steel that document carries the grade promise, the heat identity, the dimensional class and the test results, and it is the only evidence a buyer has that the material matches the order. Reading it before the plate is released into production takes minutes and prevents the expensive kind of rework.

What the grade number is promising

Chinese carbon structural grades are named for their yield floor, which makes them easier to read than most designation systems. Hot rolled steel plate Q195 carries a minimum yield of 195 MPa, Q235 the next step up, and hot rolled steel plate Q345 a higher strength again. That number is a floor rather than a target, so a certificate showing yield comfortably above the minimum is normal, because mills aim above the limit to protect against scatter. Q195 is the softest of the three and turns up in pipe, tube, scaffolding and light formed parts, where weldability and formability matter more than strength, and its corrosion resistance is limited, so it is specified indoors or with a coating. Hot rolled steel plate Q235 is the general structural workhorse, chosen for its combination of weldability and toughness. Q345 is the choice where a member is limited by strength rather than stiffness, because the higher yield allows a thinner section for the same load. All three are recycled at end of life, which matters on projects with a recovery target.

The chemistry line matters more than the strength

Yield strength is the figure buyers look at and rarely the one that causes trouble. Weldability is governed by composition, principally carbon and manganese, with sulphur and phosphorus held low because they drive brittleness and directional weakness. A plate that meets its yield requirement can still be awkward to weld if the carbon equivalent sits at the top of the range, so shops running thick sections under high restraint should ask for the full analysis rather than the mechanical line alone. The same logic runs across the rest of the family: the plain carbon grades sit at the easy-welding end, while hot rolled steel plate 45 and the 16Mn plate sit at the other, where preheat and a qualified procedure matter. Impact behaviour is the second item that hides behind a grade: quality suffixes and their attached temperature decide whether the material has been Charpy tested and at what temperature, and plate for a cold climate or a dynamically loaded member should carry that figure explicitly.

Dimensions, and which tolerance applies

Hot rolled plate is ordered across an unusually wide envelope, from 0.3 mm to 200 mm in thickness, in coil weights of 3 to 18 tonnes, and at widths cut to order with 1000, 1250 and 1500 millimetres the regular stock sizes. That breadth is exactly why the certificate has to state which tolerance class the material was rolled and measured to. Plate is conventionally split by thickness into thin plate below 4 millimetres, medium plate from 4 to 60 millimetres and extra thick plate from 60 to 115 millimetres, and the width bands differ between sheet and plate as well, roughly 500 to 1500 millimetres for sheet against 600 to 3000 millimetres for plate, as set out in how steel plate is divided into hot rolled and cold rolled. Tolerance is where buyers get caught, because the number quoted depends on the product being discussed: cold rolled sheet is commonly quoted at plus or minus one percent, while galvanized coil rolled to a Japanese standard is held to plus or minus 0.02 millimetres. Those are different classes of product, not competing claims, and an enquiry that says only "standard thickness tolerance" invites whichever reading suits the supplier. The full carbon programme sits under hot rolled steel.

Heat identity and traceability

A certificate is only useful if it can be tied to the steel in front of you. Every coil and roll carries its own identifying number, and that number is the link back to the heat and its test record, so the first check on delivery is that the tag on the bundle matches the certificate and that both match the delivery note. The scale makes identification real work rather than paperwork: a single bundle can weigh around 28.77 tonnes, and stock is held in thousands of tonnes a month. Bundles travel by road on purpose-built brackets that stabilise the load, a handling detail rather than a specification one, but transit damage is a common source of claims and the delivery condition should be recorded alongside the numbers.

Equivalent grades and substitutions

Certificates arrive written against whichever standard the mill produces to, and a project designed in one system is frequently supplied from another. ASTM A36 is the North American grade most buyers meet and is treated as the structural counterpart to Q235; ST37 is the European designation that appears on machinery and general engineering work; SS400 and the ASTM grades turn up on the same paperwork when a mill runs to Japanese or American schedules. Substitution is normal and usually sound, but it should be a documented decision rather than an assumption, because the documents do not promise identical things: minimum yield, impact requirement and chemistry limits can all differ slightly even where grades are described as equivalent. Where a fabricator has qualified a welding procedure against a named standard, the certificate has to support that procedure, and this is the check that catches most bad substitutions. The A36 side of the comparison is covered in what to know about hot rolled A36 plate in construction and on the A36 plate listing, with the ST37 route on ST37 plate.

What to ask for before dispatch

The practical version is a short list sent with the order rather than raised at delivery: the standard including its year, the grade with its quality suffix, the thickness range and tolerance class, the heat or lot number, and the mechanical results for yield, tensile and elongation. Add impact testing at a stated temperature where the service or the design code requires it, and third-party inspection where the project mandates it. Then confirm the surface condition and whether the material is supplied as-rolled, and whether processing is included, because cutting, shearing, bending, punching, welding and surface treatment are priced and scheduled separately from the plate itself. Buyers who need a non-standard size or a finished part rather than a mill bundle should raise it at enquiry stage, since custom processing is quoted as its own line.

A certificate records four promises: what the steel is, how strong it is, how accurately it was rolled, and which heat it came from. Every dispute about plate quality is ultimately a dispute about one of those four, and checking them at the gate is faster than checking them after the plate has been cut.