Q355B serves as the upgraded successor to Q345B; introduced in accordance with the Chinese standard GB/T1591-2018, it is designed to align with international standards such as S355JR. Its primary distinctions lie in a higher minimum yield strength (≥ 355 MPa, compared to ≥ 345 MPa for Q345B), superior low-temperature impact toughness, and a slightly lower carbon content-characteristics that significantly enhance both its weldability and structural performance.
What Is Q355B?
Q355B is a Chinese-produced low-alloy high-strength structural steel defined by the GB/T 1591-2018 standard. Its defining characteristic is a minimum yield strength of 355 MPa. As a modern replacement for older grades such as Q345B, it is widely utilized in critical engineering applications, offering superior strength, ductility, and excellent weldability.
Meaning of the Designation:
Q: Represents the "Yield Point"-specifically, the yield strength.
355: Indicates a minimum yield strength of 355 MPa (applicable to thicknesses less than 16 mm).
B: Denotes the quality grade; specifically, Q355B requires an impact test to be conducted at 20°C.
Equivalent Standards: Typically considered equivalent to international standards such as EN 10025-2 S355JR or ASTM A572 Grade 50.
What Is Q345B?
Q345B is a widely used Chinese low-alloy, high-strength structural steel, characterized by a minimum yield strength of 345 MPa, along with excellent comprehensive mechanical properties, weldability, and low-temperature toughness. This steel is extensively utilized in fields such as construction, machinery, bridge building, and pressure vessels, and is typically employed in the form of steel plates or pipes.
Meaning of the Name:
"Q" stands for yield strength (derived from the Pinyin "Qu").
"345" indicates the minimum yield strength (345 MPa).
"B" represents the quality grade (Grade B).
Equivalent Standards: Roughly equivalent to ASTM A572 Grade 50 and European S355JR+AR.
Chemical Composition Difference Between Q355B and Q345B
Although both possess similar basic constituents, Q355B has been optimized to exhibit superior strength and toughness, along with fewer welding restrictions; in structural design, this often permits the use of thinner steel sections (potentially reducing material usage by up to 10%).
| Element | Q345B (%) | Q355B (%) |
|---|---|---|
| Carbon (C) | ≤ 0.20 | ≤ 0.20 |
| Silicon (Si) | ≤ 0.55 | ≤ 0.50 |
| Manganese (Mn) | ≤ 1.60 | ≤ 1.60 |
| Phosphorus (P) | ≤ 0.035 | ≤ 0.025 |
| Sulfur (S) | ≤ 0.035 | ≤ 0.025 |
| Other (Nb, V, Ti) | Added optionally | Required (total ≤ 0.22) |
Mechanical Properties Difference Between Q355B and Q345B
Q355B is the modern, upgraded successor to Q345B (in accordance with the GB/T 1591-2018 standard), featuring a higher yield strength (≥ 355 MPa, compared to ≥ 345 MPa for Q345B), superior toughness, and improved weldability.
| Property | Q345B | Q355B |
|---|---|---|
| Yield Strength | ≥ 345MPa | ≥ 355MPa |
| Tensile Strength | 450–630MPa | 490–630MPa |
| Elongation | ≥ 21% | ≥ 21% |
| Impact Energy(20°C) | ≥ 27J(or 34J) | ≥ 27J(Higher toughness) |
Applications Difference Between Q355B and Q345B
Q355B (Standard GB/T1591-2018): Best suited for high-performance projects, complex welding operations, and structures requiring superior fatigue resistance. Widely applied in:
Large-scale cranes and hoisting machinery.
High-rise buildings and complex steel structures.
Long-span bridges.
Heavy-duty transport vehicles and mining equipment.
Q345B (Standard GB/T1591-2008): Suitable for conventional projects that require good overall performance while being cost-sensitive. Applied in:
General industrial structural components.
Gas pipelines and low-pressure vessels.
General machinery manufacturing.
Q345B vs. Q355B Structural steel :How to Choose?
✅ Project Requirements: Given its exceptional load-bearing capacity, Q355B steel should be selected for high-rise buildings, cranes, and heavy machinery projects.
✅ Environmental Conditions: Q355B serves as a more ideal choice for projects situated in cold environments or those requiring superior structural impact toughness.
✅ Standard Compliance: If a project adheres to the latest engineering technical standards, the use of Q355B-rather than Q345B-is a mandatory requirement.
✅ Cost-Effectiveness: For projects that prioritize the control of initial material costs over the reduction of structural self-weight, Q345B remains a reliable option.
If you require further details regarding Q355B/Q345B structural steel, please do not hesitate to contact us.
As a leading manufacturer of Q355B/Q345B structural steel in China, GNEE boasts 20 years of extensive industry experience. Our warehouses maintain a standing inventory of over 50,000 tons year-round, ensuring that orders can be dispatched in as little as seven days. Regarding quality assurance, we are a dual-certified supplier holding both API and ISO certifications, and we welcome inspections by third-party agencies such as SGS, BV, and Lloyd's.
By choosing our Q355B/Q345B structural steel products, you will benefit from our comprehensive, customer-centric support services, as well as our extensive value-added processing capabilities. We would be delighted to provide full-scale support and assistance for your project.
| Type | Q355B |
| Standard | AISI, ASTM, BS, GB, DIN, JIS, etc |
| Certificate | ISO,SGS,CE |
| Length |
Production according to customer requirements |
| Thickness | 8-160MM |
| Width | 2000-2500MM |
| Surface | Coated |
| Technique | Hot rolled |
| Package | Available on request or Standard Export Packing |
| Application | Widely used in Building Construction, Bridge, Architecture, Vehicles Components,hipping,High pressure container, Boiler, Large Structure steel etc. |
What is the grade Q345B equivalent to?
Q345B is a Chinese low-alloy high-strength structural steel, most commonly replaced by ASTM A572 Grade 50 (USA) and S355JR (EN 10025-2, Europe) due to similar yield strength and chemical composition. Other equivalents include DIN 17100 St52-3 (Germany), JIS G3101 SS490 (Japan), and Q355B (updated Chinese standard).
What is the difference between Q345B and Q345C?
Q345B: Impact test is carried out at room temperature of 20°C. Q345C: Impact test is carried out at 0°C. The lower the impact temperature, the better the low - temperature performance of the steel, which means it can maintain better toughness and ductility at low temperatures, reducing the risk of brittle fracture.
What is the yield strength of Q345 steel?
Q345 steel is a low alloy high strength structural steel with a density of 7.85 g/cm3, tensile strength of 470-630 MPa, and yield strength of 345 MPa, available in five quality grades.