Q355 is a low-alloy high-strength structural steel standardized under GB/T 1591-2018, the latest Chinese national standard for high-strength low-alloy structural steels. Introduced to replace the longstanding Q345 grade effective February 1, 2019, Q355 represents a significant generational upgrade in China's structural steel family.
The designation follows Chinese naming conventions: "Q" derives from the Mandarin Qufu (meaning "yield"), and "355" denotes the minimum guaranteed yield strength of 355 MPa for plates ≤16 mm in thickness, with a tensile strength range of 470–630 MPa. The material density is 7.85 g/cm³.
Q355 is supplied in multiple delivery conditions, including hot-rolled (AR or +AR), normalized (+N), and thermomechanically rolled (M or TMCP), with the appropriate suffix appended to the grade designation.
Q355B, Q355C, Q355D, and Q355E - designated by the final letter - represent increasing quality grades. The primary differentiator among grades is the impact test temperature and associated minimum absorbed energy requirement. Q355B is tested at +20°C, Q355C at 0°C, Q355D at −20°C, and Q355E at −40°C, all with a minimum longitudinal impact energy of 34 J
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Chemical Composition of Q355 STEEL
| Element | Q355B (wt% max) | Q355C (wt% max) | Q355D (wt% max) | Q355E (wt% max) |
|---|---|---|---|---|
| C | 0.20 | 0.20 | 0.20 | 0.18 |
| Si | 0.55 | 0.55 | 0.55 | 0.50 |
| Mn | 1.60 | 1.60 | 1.60 | 1.60 |
| P | 0.035 | 0.030 | 0.025 | 0.025 |
| S | 0.035 | 0.030 | 0.025 | 0.020 |
| Cr | 0.30 | 0.30 | 0.30 | 0.30 |
| Ni | 0.30 | 0.30 | 0.30 | 0.30 |
| Cu | 0.40 | 0.40 | 0.40 | 0.40 |
Core Mechanical Properties: Q355B Through Q355E
A systematic examination of the mechanical properties reveals both the shared strength characteristics of the Q355 family and the critical toughness differentiation across quality grades.
Yield Strength Variation with Thickness
The minimum yield strength of Q355 decreases progressively as plate thickness increases - a characteristic common to all hot-rolled structural steels due to the reduced rolling reduction and consequently coarser grain structure in thicker sections. The values are identical across all quality grades (B through E):
| Thickness Range (mm) | Yield Strength, ReH (MPa, min) |
|---|---|
| d ≤ 16 | 355 |
| 16 < d ≤ 40 | 345 |
| 40 < d ≤ 63 | 335 |
| 63 < d ≤ 80 | 325 |
| 80 < d ≤ 100 | 315 |
| 100 < d ≤ 150 | 295 |
| 150 < d ≤ 200 | 285 |
| 200 < d ≤ 250 | 275 |
Data sourced from GB/T 1591-2018 standard specifications
Tensile Properties and Elongation
The ultimate tensile strength and minimum elongation requirements are consistent across Q355B, Q355C, Q355D, and Q355E.
| Property | Value |
|---|---|
| Tensile Strength, Rm (MPa) | 470–630 |
| Elongation after Fracture, A (%) | ≥22 |
Data compiled from multiple industry sources
The elongation requirement of ≥22% demonstrates that Q355 possesses excellent ductility, allowing significant plastic deformation before fracture. This property is critical for energy absorption under dynamic and seismic loading conditions and ensures adequate warning before structural failure.
Impact Toughness: The Defining Differentiator
The impact toughness requirement - specifically the test temperature at which the material must absorb a minimum of 34 J in the longitudinal Charpy V-notch test - is the single most important characteristic distinguishing Q355B, Q355C, Q355D, and Q355E. This property directly governs low-temperature service suitability.
| Grade | Impact Test Temperature | Min Absorbed Energy (Longitudinal) | Typical Application Climate |
|---|---|---|---|
| Q355B | +20°C | ≥34 J | Indoor / mild climate |
| Q355C | 0°C | ≥34 J | Temperate outdoor |
| Q355D | −20°C | ≥34 J | Cold region / northern China |
| Q355E | −40°C | ≥34 J | Arctic / severe cold region |
Table compiled from multiple industry specifications
The practical significance of these impact requirements cannot be overstated. Q355E, tested at −40°C with a minimum impact energy of 34 J, maintains ductile fracture behavior under conditions where lower grades would undergo a ductile-to-brittle transition. This makes Q355E the material of choice for bridge structures in northern Heilongjiang, offshore platforms in the Bohai Sea, wind turbine towers in Inner Mongolia, and other extreme cold-region infrastructure.
Consolidated Mechanical Properties Table
The following table presents a comprehensive overview of the key mechanical properties for all four Q355 grades.
| Grade | Yield Strength ReH (MPa, ≤16 mm) | Tensile Strength Rm (MPa) | Elongation A (%) | Impact Test Temp. | Impact Energy KV₂ (J, min) |
|---|---|---|---|---|---|
| Q355B | ≥355 | 470–630 | ≥22 | +20°C | ≥34 |
| Q355C | ≥355 | 470–630 | ≥22 | 0°C | ≥34 |
| Q355D | ≥355 | 470–630 | ≥22 | −20°C | ≥34 |
| Q355E | ≥355 | 470–630 | ≥22 | −40°C | ≥34 |
Table synthesized from GB/T 1591-2018 specifications and multiple industry data sources
Q355 vs. Q345 vs. 16Mn
To fully appreciate the performance advantages of Q355, it is essential to position it within the evolutionary lineage of Chinese structural steels. The progression from 16Mn (GB 1591-88) to Q345 (GB/T 1591-1994/2008) to Q355 (GB/T 1591-2018) represents decades of metallurgical refinement and international standard harmonization.
Historical Evolution
16Mn, the "grandfather" grade under the now-obsolete GB 1591-88, was the workhorse structural steel in China for decades with a typical carbon content of 0.12–0.20% and manganese content of 1.20–1.60%.It was superseded by Q345 under GB/T 1591-1994, which unified multiple legacy grades (12MnV, 14MnNb, 18Nb, 16MnRE, etc.) under a single classification system.
Q345 was itself replaced by Q355 in 2019, bringing Chinese standards into closer alignment with European EN 10025 specifications (notably the S355 series).
Key Performance Differences
Yield Strength: Q355 delivers a minimum yield strength of 355 MPa for thin sections, representing a 10 MPa (approximately 3%) improvement over Q345's 345 MPa and a 10–80 MPa increase over 16Mn's thickness-dependent range of 275–345 MPa. While 10 MPa appears modest, in large-scale structural design it translates directly to reduced section dimensions and measurable weight savings.
Tensile Strength: Q355 exhibits a tensile strength range of 470–630 MPa, compared with 16Mn's 470–660 MPa and Q345's comparable range. The upper bound has been slightly tightened, reflecting improved consistency in modern steelmaking practices.
Elongation: All three grades share comparable elongation values of approximately 21–22%, indicating that the strength improvements in Q355 have been achieved without sacrificing ductility - a non-trivial accomplishment in physical metallurgy.
Low-Temperature Toughness: This is Q355's most decisive advantage. While Q345E could achieve −40°C impact performance, Q355 achieves this through a fundamentally different metallurgical pathway - microalloying with V, Ti, and Nb - which refines the grain structure rather than relying solely on compositional cleanliness. The Q355E grade delivers −40°C impact toughness with ≥34 J absorbed energy, a capability that legacy 16Mn simply could not offer
Conclusion
Q355 hot-rolled steel, encompassing grades B through E, represents the current state of the art in Chinese low-alloy high-strength structural steel technology. Its mechanical property profile - minimum yield strength of 355 MPa, tensile strength of 470–630 MPa, elongation ≥22%, and grade-specific impact toughness extending to −40°C - establishes it as a versatile, high-performance material suitable for the most demanding structural applications.
FAQ
What are the mechanical properties of Q355?
Mechanical Properties
Generally, Q345 steel has a tensile strength range of 470-630 MPa, while Q355 steel has a tensile strength range of 470-680 MPa. This indicates that Q355 has a potentially higher tensile strength than Q345.
What is the difference between S355 and Q355?
Q355 and S355 are not identical standards, but they are functionally equivalent for most structural applications. Both have a minimum yield strength of 355 MPa and similar tensile strength ranges. The main differences lie in standard specifications, chemical composition limits, and delivery conditions
What is Chinese steel grade Q355?
Q355 steel is a Chinese structural steel grade that is part of the GB/T 1591 standard. It is a high-strength low-alloy steel known for its excellent mechanical properties, making it suitable for various structural applications in construction and engineering.
What is the difference between Q345 and Q355?
The main difference between Q345 and Q355 is their properties and yield strength. Q355 has higher strength and a more comprehensive range of applications, while Q345 is a more common low-alloy steel.
What is Q355 steel equivalent to?
Q355 steel is generally considered equivalent to the EN S355 structural steel series in terms of yield strength and mechanical performance. The closest European equivalents are S355JR, S355J0, and S355J2, depending on impact test temperature requirements.
