What Q355 Steel Is
Q355 is a low-alloy high-strength structural steel defined by the Chinese national standard GB/T 1591-2018, which covers weldable fine grain structural steels. In the designation the letter Q stands for the Chinese word for yield strength and the number 355 gives the minimum yield strength in megapascals at the reference thickness of 16 mm and below. The grade replaced Q345 when GB/T 1591 was revised in 2018, and current design and procurement documents in China refer to Q355.
Compared with plain carbon structural steel such as Q235B to GB/T 700-2006, Q355 delivers roughly fifty percent more yield strength at a similar carbon level, achieved through micro-alloying and controlled rolling rather than through higher carbon. That is the reason it can be welded with conventional procedures while carrying significantly higher design loads.
Grade Suffixes and Impact Toughness
The quality suffix letter states the Charpy V-notch impact test temperature: Q355B at +20 degrees C, Q355C at 0 degrees C, Q355D at -20 degrees C and Q355E at -40 degrees C. GB/T 1591-2018 sets the minimum absorbed energy at 34 J for these impact tests, which is a more demanding requirement than the 27 J used by the equivalent European quality classes.
Selection of the suffix is a service-temperature decision, not a strength decision. All four letters give the same minimum yield strength at the same thickness, so a cold-store structure or an outdoor installation in a cold region should be ordered as D or E regardless of the load level.
Mechanical Properties by Thickness
GB/T 1591-2018 reduces the guaranteed minimum yield strength as the section becomes thicker, because through-thickness cooling rate and rolling reduction change the microstructure.
| Thickness range (mm) | Minimum yield strength ReH (MPa) | Tensile strength Rm (MPa) |
|---|---|---|
| Up to 16 | 355 | 470-630 |
| Over 16 to 40 | 345 | 470-630 |
| Over 40 to 63 | 335 | 470-630 |
| Over 63 to 80 | 325 | 470-630 |
| Over 80 to 100 | 315 | 470-630 |
The tensile strength band of 470-630 MPa is wide enough to accommodate both hot-rolled and controlled-rolled product, and the purchaser should confirm the delivery condition on the mill certificate because it affects toughness and formability as well as strength.
Chemical Composition and Carbon Equivalent
GB/T 1591-2018 keeps carbon low and relies on manganese with small additions of micro-alloying elements to reach the strength level. Carbon equivalent value and, for higher grades, the crack sensitivity index are limited by the standard, with the permitted maxima becoming tighter as the quality class increases from B to E. The purpose is to preserve the toughness of the heat-affected zone after welding.
Practical welding follows from that chemistry: preheat is generally modest for thin sections and rises with thickness and with the measured carbon equivalent value. Low-hydrogen consumables and controlled heat input are standard practice for restrained joints, and welding procedures are normally qualified to ISO 15614-1 with consumables classified under ISO 14341 or ISO 14171 as applicable.
Q355 Versus the Older Q345 Grade
The change from Q345 to Q355 in GB/T 1591-2018 was primarily a tightening of the yield strength guarantee: the older Q345 series guaranteed 345 MPa at the reference thickness, while Q355 guarantees 355 MPa. Steel ordered to the 2018 edition therefore provides a small strength increase, and the composition and carbon equivalent limits were also adjusted. Projects that still reference the older grade should confirm which edition applies, because substitution without checking can affect both strength calculations and welding procedure qualification.
Structural design in China is carried out to GB 50017, and the design strength values in that code are aligned with the grade series in GB/T 1591.
Typical Applications and Procurement Notes
Q355 is used for building and industrial frames, crane girders, transmission towers, bridges, mining equipment, vehicle chassis, storage tanks and general welded machinery. It is supplied as plate, strip, bar, and as rolled sections including angles, channels and H sections.
An order should state the grade letter including the impact suffix, the delivery condition, the plate or section size, and the required certificates. Typical verification covers chemical analysis, tensile testing and impact testing on a sample taken in accordance with the standard, reported to EN 10204 type 3.1 with heat number traceability.
Frequently Asked Questions
Q: What does the number 355 in Q355 mean?
A: It is the minimum yield strength in megapascals for thickness up to 16 mm, which is the reference thickness used in GB/T 1591-2018.
Q: What is the difference between Q355B and Q355D?
A: Only the impact test temperature differs. Q355B guarantees 34 J at +20 degrees C while Q355D guarantees the same energy at -20 degrees C. Yield and tensile strength requirements are the same.
Q: Is Q355 the same as S355?
A: They are close but not identical. Q355 is governed by GB/T 1591-2018 and S355 by EN 10025-2, and the two standards differ in composition limits and in impact energy requirements, so substitution should be checked against the design specification.
Q: Can Q355 be welded without preheat?
A: For thin sections with a low carbon equivalent value, welding with a minimal preheat is common practice. Thicker sections and higher carbon equivalent values require preheat and controlled heat input, with the procedure qualified by test.
Q: Is Q355 available in a quenched and tempered condition?
A: GB/T 1591-2018 covers the weldable fine grain grades. Where a higher strength quenched and tempered plate is required, grades such as Q690 are specified to the separate standard for high strength plates in the quenched and tempered condition.
Q: Does the grade still carry the old Q345 marking?
A: Material produced to the earlier edition of GB/T 1591 may be marked Q345. Documents should state which edition of the standard applies so that certificates and design checks are aligned.