Q960D is a Chinese ultra-high-strength structural steel grade defined in GB/T 16270, the standard for high-strength structural steel plates supplied in the quenched and tempered condition. The designation follows the Chinese naming system: Q indicates a yield-strength-guaranteed steel, 960 is the minimum yield strength in megapascals, and D indicates Charpy V-notch impact testing at -20 deg C. The grade is used in weight-critical structures such as crane booms, excavator components, and heavy equipment frames.
Chemical Composition
Composition limits for Q960D are specified in GB/T 16270 for ladle analysis and vary with plate thickness and the edition of the standard. Typical maximum limits follow this pattern:
| Element | Typical maximum, % |
|---|---|
| Carbon (C) | 0.18 |
| Silicon (Si) | 0.60 |
| Manganese (Mn) | 1.80 |
| Phosphorus (P) | 0.025 |
| Sulfur (S) | 0.015 |
| Chromium (Cr) | 1.50 |
| Nickel (Ni) | 1.50 |
| Molybdenum (Mo) | 0.70 |
| Copper (Cu) | 0.50 |
| Boron (B) | 0.004 |
| Niobium, Vanadium, Titanium (Nb, V, Ti) | Microalloy additions, each with its own limit |
The exact limits depend on the applicable edition of GB/T 16270 and the ordered thickness, so the values on the mill test certificate supplied with the material are authoritative for verification.
Role of the Alloying Elements
Carbon and manganese provide base strength, with carbon limited to keep the steel weldable.
Chromium, nickel, and molybdenum increase hardenability so that the quenched microstructure is uniform through the plate thickness.
Boron is added in small amounts to boost hardenability economically.
Niobium, vanadium, and titanium refine the grain structure and contribute precipitation strengthening.
Phosphorus and sulfur are tightly controlled to maintain toughness and internal cleanliness.
The carbon equivalent (CEV) is held at a low level to reduce the risk of cold cracking during welding; industry data for the grade commonly cite values around 0.48%, and the applicable limit is confirmed on the mill test certificate.
Mechanical Properties
Yield strength (ReH): minimum 960 MPa for thicknesses up to 50 mm, decreasing for thicker plates as specified in GB/T 16270.
Tensile strength (Rm): 980-1150 MPa for the 50 mm thickness range.
Elongation (A): minimum 10% for the 50 mm thickness range, with values for other thicknesses per the standard.
Impact energy: Charpy V-notch testing at -20 deg C for the D designation, with the minimum absorbed energy stated in the standard and on the mill certificate.
The strength is developed by a quenched and tempered microstructure, typically fine lath martensite with bainite, obtained by controlled rolling followed by quenching and tempering. This gives a high strength-to-weight ratio, which is the main reason the grade is used to reduce section sizes and equipment weight.
Production Route
Q960D plates are produced through a controlled route: converter or electric arc furnace melting with secondary refining and vacuum degassing to remove impurities and gases, continuous casting into slabs, controlled rolling and controlled cooling to form a fine microstructure, and quenching and tempering heat treatment to reach the final property level. Surface and internal quality are verified by non-destructive testing such as ultrasonic examination where the specification or order requires it.
Cutting, Machining, and Welding Notes
Plates thinner than about 20 mm: laser or plasma cutting is recommended for dimensional accuracy.
Plates thicker than about 30 mm: flame cutting with preheating to 150-200 deg C helps prevent hardening of the heat-affected zone.
Machining: carbide tooling with lubrication is recommended, and high-speed drilling should be avoided because it can work-harden the surface.
Welding of Q960D requires a qualified welding procedure specification, low-hydrogen consumables, preheat and interpass temperature control, and management of heat input. Post-weld heat treatment is generally avoided for this grade unless the design code specifically requires it.
Frequently Asked Questions
What does Q960D stand for? Q denotes a yield-strength-guaranteed structural steel, 960 is the minimum yield strength in megapascals, and D indicates Charpy V-notch impact testing at -20 deg C under GB/T 16270.
What is the yield strength of Q960D? The minimum yield strength is 960 MPa for thicknesses up to 50 mm, decreasing for thicker plates as specified in GB/T 16270.
What is the difference between Q960C, Q960D, and Q960E? The grades share the same strength level but differ in impact test temperature: C is tested at 0 deg C, D at -20 deg C, and E at -40 deg C. The colder test temperature indicates a tougher grade for lower-temperature service.
Can Q960D be welded? Yes, but only with strict process control: a qualified welding procedure, low-hydrogen consumables, preheat and interpass temperature limits, and controlled heat input. The low carbon equivalent is designed to keep the risk of cold cracking manageable.
What are Q960D plates used for? They are used for weight-critical structural components such as crane booms, excavator arms, rotary drilling rig winch drums, heavy transport frames, and other equipment where high strength-to-weight ratio is essential.
How does Q960D compare with S960QL? Q960D is the Chinese grade to GB/T 16270, while S960QL is the European grade to EN 10025-6. Both are quenched and tempered steels with about 960 MPa minimum yield strength; the L suffix of S960QL guarantees impact testing at -40 deg C. They serve similar applications but are governed by different standards, so substitution requires verifying all required properties.