Q620D and Q620E are two variants of high-strength low-alloy structural steels under Chinese national standards. They share the same minimum yield strength of 620 MPa, as well as desirable weldability and formability. Yet the disparity in low-temperature impact toughness requirements is the core differentiator, which in turn brings about variations in material properties, manufacturing processes and application scopes.
Core Distinction
The letters "D" and "E" denote different quality grades, with the essential difference lying in the temperature conditions and toughness benchmarks for impact testing. This parameter is the key determinant for their application scenarios.
| Steel Grade | Impact Test Temperature | Impact Energy Requirement |
|---|---|---|
| Q620D | -20℃ | It needs to maintain sufficient impact energy to prevent brittle fracture at this temperature, making it ideal for moderately cold operating conditions |
| Q620E | -40℃ | The Charpy V-notch impact energy is required to be no less than 27 J, and in many engineering practices, the measured value can even surpass 47 J. It is capable of withstanding severe ultra-low temperature environments and avoiding structural failure in frigid conditions |
Minor Adjustments in Chemical Composition
The two steels have similar base chemical compositions, with carbon and manganese as the main strengthening elements, supplemented by microalloying elements like niobium, vanadium and titanium for grain refinement. However, Q620E imposes stricter composition control to meet the toughness demands at lower temperatures:
- Q620D: The content of detrimental elements such as phosphorus and sulfur is controlled to regular standards, only meeting the purity requirements of general high-strength steels. No special alloy ratio adjustment is needed for ultra-low temperature service.
- Q620E: Besides limiting phosphorus and sulfur to ultra-low levels, it optimizes the proportion of alloying elements including chromium, molybdenum and nickel. Meanwhile, the carbon equivalent (Ceq ≤ 0.48%) is accurately controlled, which ensures high strength while enhancing toughness at -40℃ and preventing low-temperature embrittlement.
Divergent Manufacturing Processes
Both steels go through standard procedures like smelting, rolling and heat treatment, butQ620E requires more sophisticated process control to meet its low-temperature performance targets:
- Q620D: It is mostly manufactured via hot rolling or conventional quenching and tempering processes. The focus is placed on controlling rolling temperature and deformation to achieve uniform internal microstructure, which only needs to meet the basic toughness standard at -20℃.
- Q620E: It is usually produced with the Thermo-Mechanical Control Process (TMCP). After rolling, Accelerated Cooling Control (ACC) is employed to precisely adjust the cooling rate. In some cases, an additional normalizing treatment at 900–950℃ is required to eliminate residual stress. These measures help obtain a duplex microstructure of fine-grained ferrite and bainite, ensuring stable performance in extremely cold environments.
Specialized Application Scenarios
Based on their different low-temperature performance, the two steels are applied in distinct scenarios: Q620E is designed for extreme cold conditions, while Q620D is suited for moderately cold or normal temperature environments.
- Q620D: It is widely used in oil and gas transmission pipelines, general power plant boiler components, construction machinery structural parts, as well as load-bearing components of bridges and industrial buildings in temperate and subtropical regions. It can handle routine low temperatures but is not intended for extremely cold conditions.
- Q620E: It is applicable to ultra-low temperature environments such as high-latitude frigid regions and deep seas. Typical applications include the -45℃ section of the China-Kazakhstan Eastern Route Natural Gas Pipeline, polar LNG storage tanks, low-temperature related pipelines of ultra-supercritical power plants, and jacket structures of deep-sea drilling platforms. It can maintain long-term structural safety in harsh cold conditions.
Cost and Testing Requirements
- Cost: Owing to its optimized alloy formulation and complex manufacturing process, Q620E has higher production costs and generally commands a higher market price compared to Q620D.
- Testing: Q620E requires additional -40℃ low-temperature impact tests, and in some projects, more rigorous non-destructive testing methods such as ultrasonic flaw detection are needed to ensure no internal defects that may affect low-temperature performance. In contrast, Q620D only needs to pass the -20℃ impact test and routine quality inspections.
Can Q620D be used as a substitute for Q620E in projects located in frigid regions?
Substitution is not recommended. Frigid regions usually experience long-term ultra-low temperatures below -20℃, and even as low as -40℃ in some areas. Q620D only meets the impact toughness requirement at -20℃; its performance deteriorates sharply in environments below this temperature, which can easily lead to structural brittle fracture. Q620E is specifically engineered for ultra-low-temperature conditions, with a Charpy V-notch impact energy of no less than 27J at -40℃, and actual project test values can even exceed 47J. For instance, it has been successfully applied in the -45℃ section of the China-Kazakhstan Eastern Route Natural Gas Pipeline, ensuring long-term structural safety in extreme cold.
What are the typical application scenarios for Q620D and Q620E respectively?
Q620D is widely used in temperate and subtropical regions, for applications such as oil and gas transmission pipelines, general power plant boiler components, construction machinery structural parts, and load-bearing components of bridges and industrial buildings. It is suitable for routine low-temperature conditions but not for extreme cold. Q620E is designed for high-latitude frigid regions and deep-sea ultra-low-temperature environments, with typical applications including polar LNG storage tanks, low-temperature pipelines of ultra-supercritical power plants, and jacket structures of deep-sea drilling platforms.
Do Q620D and Q620E have the same minimum yield strength?
Yes, they do. Both Q620D and Q620E belong to the 620MPa grade high-strength low-alloy structural steels, with a minimum yield strength of 620MPa. Their core difference lies in low-temperature impact toughness rather than basic strength performance.