Q355 steel: properties, composition, equivalents

Q355 is the Chinese low-alloy structural steel of GB/T 1591: 355 MPa yield to 16 mm, 470–630 MPa tensile, in quality classes B, C and D. It replaced Q345 in 2018.

Last updated 26 September 2026

What is Q355?

Q355 is the Chinese low-alloy structural steel that carries the load wherever Q235 is not strong enough — bridges, heavy frames, crane runways, offshore modules. It is defined in GB/T 1591, the Chinese HSLA standard, and it is the grade most often offered against an S355 enquiry from Asian supply.

The name follows the same rule as Q235: Q is *qufu dian*, the yield point, and 355 is that yield in MPa up to 16 mm. The letter after it — B, C or D — grades the impact test and not the strength, which is identical across all three.

The single most important fact about this grade is a date. Q355 did not exist before 2018. The 2018 revision of GB/T 1591 replaced Q345 with Q355, raising the guaranteed yield by 10 MPa and bringing the grade into line with the international 355 MPa class. Drawings, catalogues and stock lists across the industry still say Q345, and mills supply Q355B against them. A certificate for Q345 issued after 2018 is working from a withdrawn edition, and the 2018 revision also withdrew quality class A — there is no Q355A.

Against S355 the match is unusually clean. Same yield at the first step, the same 470–630 MPa tensile band, and a quality ladder that maps one to one onto JR, J0 and J2. Two differences deserve attention: the Chinese impact requirement is 34 J where the European is 27 J, and the thickness ladders diverge above 63 mm.

Q355 chemical composition

GB/T 1591-2018 ladle analysis for the as-rolled Q355 grades. Carbon, phosphorus and sulphur all tighten with the quality letter, exactly as they do in the European J0 / J2 ladder. Niobium, vanadium and titanium are permitted as microalloying additions rather than required, so two conforming Q355B plates can be strengthened by quite different routes.

Q355 mechanical properties

The thickness ladder is close to EN 10025-2 but not identical: Q355 steps down at 16 mm where S355 holds 355 MPa to 16 mm and then 345 MPa to 40 mm — the two agree at the first two steps and diverge above 63 mm. The tensile band 470–630 MPa is the same as the European one, which is unusual and makes the comparison unusually clean.

Impact toughness

Q355 requires a minimum Charpy V-notch energy of 34 J at 20 °C (Q355B longitudinal; C at 0 °C, D at −20 °C). The quality letter is the impact test, as with Q235. **Q355B** is tested at +20 °C, **Q355C** at 0 °C and **Q355D** at −20 °C, each to **34 J**. The 34 J figure is the point of difference from Europe: EN 10025-2 requires 27 J for the S355 JR / J0 / J2 ladder, so a conforming Q355 is tested slightly harder at the same temperature. Quality class A was withdrawn in the 2018 revision.

Q355 equivalent grades

Identical = same composition and mechanical limits within rounding. Near = one limit differs (e.g. carbon max or impact temperature). Functional = interchangeable for most uses after an engineering check. Full cross-reference for Q355 →

Old and superseded designations

Product forms and tolerance standards

Product forms: hot-rolled plate and heavy plate, sections: H beam (GB/T 11263), channel, angle, hot-rolled coil and strip, cold-formed and welded hollow sections (GB/T 6728, GB/T 8162), round and flat bar.

Dimensional tolerances: plate and coil: GB/T 709; H beam: GB/T 11263; channel and angle: GB/T 706; round bar: GB/T 702.

Key property values

Q355 yield strength

355 MPa minimum up to 16 mm, then 345 MPa to 40 mm, 335 MPa to 63 mm, 325 MPa to 80 mm, 315 MPa to 100 mm and 295 MPa to 150 mm. The first two steps are identical to EN 10025-2; above 63 mm the European grade holds its strength slightly better.

Q355 tensile strength

470–630 MPa up to 100 mm, falling to 450–600 MPa above. This is the same band EN 10025-2 specifies for S355, which makes tensile the one property where the two standards agree exactly — and it is worth noticing that both set a ceiling, unlike GOST 19281.

Q355B, C and D

The letters grade the Charpy test. B: 34 J at +20 °C, C ≤ 0.24 %, P and S ≤ 0.035 %. C: 34 J at 0 °C, C ≤ 0.20 %, P ≤ 0.030 %, S ≤ 0.030 %. D: 34 J at −20 °C, C ≤ 0.20 %, P ≤ 0.025 %, S ≤ 0.020 %, with an aluminium minimum for fine grain. Q355B is the commercial stock grade.

Q345 became Q355

The 2018 revision of GB/T 1591 replaced Q345 with Q355 and raised the guaranteed yield from 345 to 355 MPa at the first thickness step. Quality class A disappeared at the same time. Material offered as Q345B today is almost always Q355B on the certificate; where a drawing specifies Q345, confirm which edition the design was checked against before accepting the substitution upwards.

N and M delivery conditions

The 2018 edition added Q355N (normalised or normalised-rolled) and Q355M (thermomechanically rolled) alongside the as-rolled grade, with their own chemistry and toughness tables and quality letters up to F at −60 °C. They correspond to the European EN 10025-3 and EN 10025-4 families. Plain Q355 without a suffix is the as-rolled grade.

Accepting Q355 against S355

Three checks. The quality letter must be on the certificate, not just "Q355". The impact temperature must match the design — B, C and D are +20, 0 and −20 °C. And above 63 mm the yield ladders diverge, so the governing thickness has to be checked rather than assumed. The tensile band and the 355 MPa first step need no adjustment at all.

Weldability

Q355B welds like any 355 MPa structural steel of its generation. Carbon equivalent is limited by the standard and normally runs 0.40–0.45 for plate in the common thicknesses, so preheat is unnecessary below about 25 mm and 100–150 °C is the usual figure above it. All arc processes apply, with E50xx / ER70S-class consumables. The point requiring attention is the optional microalloying: GB/T 1591 permits niobium, vanadium and titanium but does not require them, so two conforming heats can reach 355 MPa by different metallurgical routes and behave differently in the heat-affected zone. Where the joint is thick and restrained, qualify on the declared analysis rather than on the grade name, and prefer C or D where the design allows — their carbon ceiling is 0.20 % against 0.24 %.

Machining, forming and heat treatment

Machinability is moderate, comparable with S355 and noticeably harder work than Q235: expect roughly 30 % higher cutting forces than a 235 MPa steel at the same hardness. Cold forming is acceptable for the strength class, with elongation from 22 % at thin gauge down to 18 % above 80 mm, but the tensile ceiling of 630 MPa still leaves a wide band and spring-back varies between heats. For cold-formed sections and press work the better Chinese specification is the forming series of GB/T 3274 or the cold-rolled grades, not the structural one.

Typical applications

• Bridges, crane runways and heavy building frames in Chinese and export construction
• Offshore and industrial module steelwork
• Pressure-bearing structural components and heavy welded fabrications
• Mining, earthmoving and heavy vehicle structures
• Plate for silos, hoppers and large tank shells
• Sections and plate exported against an S355JR, S355J0 or S355J2 requirement

Q355 compared with related grades

Frequently asked questions

Related grades

Verified against GB/T 1591-2018; GB/T 1591-2018 — High strength low alloy structural steels; GB/T 700-2006 — Carbon structural steels, for the Q235 comparison. Last checked: September 2026.