Reading the Grade Designation
Q690E is a low-alloy high-strength structural steel plate grade issued under GB/T 1591, the Chinese standard for high strength low-alloy structural steels. The designation is built from three elements. The letter Q stands for the Chinese term for yield strength. The number 690 is the minimum yield strength in megapascals, specified for plate thickness of 16 mm and below. The letter E is the quality grade and indicates that the steel is impact tested at -40 C.
Because the yield strength requirement steps down as thickness increases, the grade designation is always read together with the plate thickness. A plate ordered at 60 mm does not carry the same minimum yield strength as a 12 mm plate, and the order, the drawing and the certificate all need to state the thickness for the strength requirement to be unambiguous.
Chemical Composition
The composition is based on a low-carbon, fine-grain steel with microalloying additions that control the transformation behaviour and the grain size.
| Element | Content (percent by mass) |
|---|---|
| Carbon | 0.18 max |
| Silicon | 0.60 max |
| Manganese | 2.00 max |
| Phosphorus | 0.025 max |
| Sulphur | 0.020 max |
| Microalloying | Nb, V, Ti additions as required |
Nickel, chromium and molybdenum can be present as required by the producer to achieve the strength and toughness combination, and the carbon equivalent is controlled so that the plate remains weldable in the thickness range supplied.
Mechanical Properties
| Property | Requirement |
|---|---|
| Minimum yield strength, t not more than 16 mm | 690 MPa |
| Tensile strength | 770-940 MPa |
| Minimum elongation | 14 percent |
| Impact test temperature | -40 C |
| Minimum Charpy absorbed energy | 31 J longitudinal |
The quality grade is what separates Q690E from the same strength level in other grades: the E class guarantees notch toughness at -40 C, which makes the plate suitable for structures that operate in cold climates and for dynamically loaded components.
Surface Quality
The plate surface must not contain cracks, bubbles, scars, laps or inclusions. Where such defects are present they may be removed by grinding or chipping, and the cleaned depth must not exceed half of the thickness tolerance. The cleaned area must be smooth and free from sharp edges, and the remaining section must still meet the minimum allowable thickness. Laminations are not permitted, and any other local defect that remains must satisfy the same depth limitation. Plates are inspected against these rules before despatch, and the results are recorded on the inspection report.
Applications
Electric power transmission pylons and substation structures
Lifting cranes, mobile cranes and crane booms
Heavy duty vehicles, dump trucks and mining automobiles
Oil derricks and drilling structures
High-temperature fans and heavy rotating equipment frames
Large electric shovels, excavators and hydraulic supports
Bridge and building structures where high strength allows section reduction
Welding, Forming and Selection Notes
The grade is welded with low-hydrogen consumables matched to the 690 MPa strength class, using procedures qualified for the actual thickness and joint type. Preheat is selected from the carbon equivalent and the section thickness, and heat input is controlled to protect the toughness of the heat affected zone. Where a post-weld heat treatment is applied, it must be carried out at a temperature below the tempering range of the base plate so that the delivered strength is not reduced.
Cold forming requires attention to springback, because high strength plate recovers more than ordinary structural steel, and bending radii are kept generous. Flame-cut edges are dressed and all surfaces are visually inspected for cracks before welding, particularly on the cut faces of thick plate.
Frequently Asked Questions
Q: What does the E in Q690E mean?
A: It is the quality grade of the steel and indicates that the plate is impact tested at -40 C.
Q: For which thickness does Q690E carry a 690 MPa minimum yield strength?
A: The 690 MPa minimum applies to plate thickness of 16 mm and below; at greater thickness the specified minimum is lower, in steps defined by the standard.
Q: Is Q690E the same as Q690D?
A: No. The strength level is the same but the impact grade differs: the D grade is tested at -20 C, while the E grade is tested at -40 C.
Q: How deep may surface defects be removed?
A: Cleaning depth must not exceed half of the thickness tolerance, and the remaining thickness must still be within the allowable range. Laminations are not permitted.
Q: Can Q690E be welded with ordinary carbon steel electrodes?
A: No. Consumables matched to the 690 MPa strength class are used, together with a qualified procedure, controlled heat input and preheat selected from the carbon equivalent.
Q: Why does thickness affect the strength requirement?
A: Because the cooling rate and the degree of centre segregation change with thickness, the standard specifies lower minimum yield strengths as plate thickness increases.
