Q1What is the difference between carbon steel plate and low alloy steel plate?
Plain carbon steel gets its strength from carbon and manganese in the as-rolled or normalized condition — A36, S235JR, Q235B, SS400 and St37-2 sit here. Low-alloy steel adds small amounts of niobium, vanadium, titanium, copper, chromium and nickel and combines them with controlled or thermomechanical rolling to get a finer grain and a higher yield from the same carbon level — A572, S355N, S355M, Q345 and the DIN 17102 StE grades sit here. The practical difference is strength per kilogram and, at the top of the range, the need for heat treatment: above roughly 690 MPa yield the plate has to be quenched and tempered (A514, EN 10025-6, GB/T 16270).
Q2Is A36 the same as S235JR, Q235B or SS400?
No. They are frequently quoted for the same kind of light structural work and they sit close together in strength, but they are separate specifications with their own chemistry limits, impact requirements, thickness coverage and dimensional tolerances. A36 is an ASTM specification quoted in ksi, S235JR is EN 10025-2 with a defined Charpy requirement, Q235B is GB/T 700 with its own quality-grade lettering, and SS400 is JIS G3101 with the number being tensile strength rather than yield. Substituting one for another is an engineering decision that has to be checked against your design code and your welding procedure.
Q3How do I choose the right strength class?
Start from what governs the design. If it is stiffness or ease of fabrication, a plain carbon grade at 235–275 MPa is usually right and the cheapest. If it is strength per kilogram — bridges, long spans, chassis, crane structures — the HSLA grades at 345–460 MPa are the standard answer. Above 500 MPa you are in quenched and tempered territory, where the welding procedure has to be qualified, preheat becomes routine, and forming is limited.
Q4Do the impact and toughness suffixes really matter?
Yes, and they are the most common source of a wrong delivery. In EN 10025-2, JR / J0 / J2 are 27 J Charpy at +20 / 0 / −20 °C and K2 is 40 J at −20 °C; N and NL, M and ML are normalized and thermomechanical pairs with NL and ML the low-temperature variants; Q, QL and QL1 are quenched and tempered with testing at −20, −40 and −60 °C. Under GB/T the trailing letter does the same job: A to E on GB/T 1591 and C to F on GB/T 16270. Ordering S355J2 when the design needs S355K2, or Q345B when it needs Q345D, gives you plate that meets the strength but not the toughness.
Q5What sizes, processing and lead time can I expect?
Plate from 1.5 mm to 200 mm thick, 1000–3000 mm wide and 2000–12000 mm long, with custom ultra-thin and ultra-thick sizes available outside that window. Processing covers cutting, bending, drilling, polishing, surface treatment and customised shaped parts to drawing. Conventional sizes are held in volume for fast delivery, and special sizes are customised within 3–7 working days. Indicative lead time is 5 days for 1–10 tonnes, 10 days for 10–50 tonnes and negotiable above 50 tonnes. Section 02 gives the full window.
Q6What information do you need for a quotation?
The standard and the full grade including any suffix letter — S355J2 rather than S355, Q345D rather than Q345 — the dimensions, quantity and any dimensional tolerance, the delivery condition if the standard offers a choice, the minimum design temperature and any Charpy or through-thickness requirement, whether the part will be formed or welded and to what procedure, the surface and edge condition, any inspection or certification needed including third-party inspection, and the destination port.
