Grade Definition
Q420D is a low-alloy high-strength structural steel plate covered by GB/T 1591. The designation records the yield strength character of the steel, the strength level and the impact quality class. Q stands for yield strength, 420 is the minimum yield strength in megapascals for plate thickness up to 16 mm, and D is the quality grade, indicating impact testing at -20 C. The quality grades in the series run from A to E, and they differ in impact test temperature rather than in strength.
The grade is intended for welded structures where ordinary carbon structural steel would require excessive section thickness. Using Q420D allows a structure to be built with thinner plate at the same load carrying capacity, which reduces weight and welding volume.
Chemical Composition
| Element | Content (percent by mass) |
|---|---|
| Carbon | 0.20 max |
| Silicon | 0.60 max |
| Manganese | 1.70 max |
| Phosphorus | 0.030 max |
| Sulphur | 0.030 max |
| Microalloying | Nb, V, Ti, Al as required |
Nickel, chromium, molybdenum and copper are permitted in controlled amounts, and the carbon equivalent is limited so that the plate can be welded with conventional processes and moderate preheat.
Mechanical Properties
| Property | Requirement |
|---|---|
| Minimum yield strength, t not more than 16 mm | 420 MPa |
| Tensile strength | 520-680 MPa |
| Minimum elongation | 18 percent |
| Impact test temperature | -20 C |
| Minimum Charpy absorbed energy | 34 J longitudinal |
Within the thickness range of the standard, the minimum yield strength is specified in steps as thickness increases, so the plate thickness must always be read together with the strength requirement. The D quality class gives the grade notch toughness at -20 C, which covers a large part of the outdoor service temperature range in temperate and northern regions.
Properties in Service
High strength with a yield to tensile ratio controlled for structural design
Good toughness and resistance to impact loading
Fatigue performance suited to repeated stress cycles in bridges and machinery
Weldability supported by controlled carbon equivalent and microalloying
Formability and machinability adequate for fabrication of plate structures
Applications
Building structures, industrial plants and large span frames
Bridge girders and highway structures
Ship and marine structures
Vehicle frames, chassis and transport equipment
Storage tanks, silos and hoppers
Crane structures, transmission towers and heavy machinery frames
Fabrication, Welding and Quality Control
The plate is welded with low-hydrogen consumables matched to the 420 MPa class. Preheat and interpass temperature are selected from the plate thickness and the carbon equivalent of the actual heat analysis, and heat input is controlled to protect toughness in the heat affected zone. A post-weld heat treatment, where required by the design, is carried out at a temperature below the tempering range of the base plate.
Cold bending is performed with attention to springback and bend radius, and flame-cut edges are dressed before welding. Plates are delivered with a material certificate showing the heat number, chemical composition, tensile and impact results and the delivery condition, and dimensional inspection covers thickness, flatness, length, width and edge condition. Surface quality is inspected against the acceptance rules of the standard, and harmful defects are removed by grinding provided the minimum allowable thickness is maintained.
Frequently Asked Questions
Q: What does Q420D mean?
A: Q indicates yield strength, 420 is the minimum yield strength in MPa for thickness up to 16 mm, and D is the quality grade with impact testing at -20 C.
Q: How does Q420D differ from Q420E?
A: The strength level is the same; the difference is the impact test temperature, which is -20 C for the D grade and -40 C for the E grade.
Q: Is the yield strength the same for all thicknesses?
A: No. The specified minimum yield strength decreases in steps as plate thickness increases, so the thickness must be considered together with the grade.
Q: Can Q420D be welded without preheat?
A: For thin plate with a low carbon equivalent, welding without preheat is often possible; for thicker sections and higher carbon equivalent, a controlled preheat is required to avoid cracking.
Q: Where is Q420D plate typically used?
A: In bridges, building structures, ships, vehicle frames, storage tanks and heavy machinery structures where weight reduction is important.
Q: What documents are supplied with the plate?
A: A material certificate showing the heat number, chemical analysis, tensile and impact results and delivery condition, together with the dimensional inspection record.
