EN 10025-6 S690Q High Strength Structural Steel Plate

EN10025-6 S690Q is a high-strength quenched and tempered structural steel that complies with the European standard EN 10025-6. It is primarily used in engineering structural applications where stringent requirements for material strength, toughness, and workability are essential.

Standard and Grade Designation

1. Standard: EN 10025-6

EN 10025: The European standard for "Hot rolled products of structural steels".

-6: Part 6 of this standard, which covers the technical delivery conditions for high yield strength structural steels in the quenched and tempered condition.

2. Grade: S690Q

S: Stands for "Structural steel";

690: Represents the specified minimum yield strength of 690 MPa;

Q: Indicates the delivery condition is Quenched and Tempered. This is a specific heat-treatment process that gives the steel its high strength and good toughness.



Key Characteristics and Properties

This steel is designed for applications where high strength-to-weight ratio is critical. Its main properties are:

Extremely High Strength: Its minimum yield strength (ReH) is 690 MPa and its tensile strength (Rm) ranges from 770 to 940 MPa. This allows for the design of much lighter and slimmer structures compared to milder steels, without sacrificing load-bearing capacity.

Good Toughness: Despite its high strength, it maintains good toughness and resistance to brittle fracture, especially at low temperatures. 

Weldability: S690Q is weldable, but it requires strictly controlled welding procedures due to its high carbon equivalent (CEV). 

Machinability: The tempered sorbite structure in the quenched and tempered state has moderate hardness, allowing machining operations such as drilling, cutting, and bending. However, high-strength tools are required for these processes.

Corrosion Resistance: Its basic corrosion resistance is comparable to that of ordinary low-alloy steel. If enhanced corrosion resistance is needed, its weathering steel variant (e.g., S690Q-W) can be selected, or surface anti-corrosion treatments (such as painting, galvanizing, etc.) can be applied.

* Heat Analysis, wt. %, maximum unless indicated

Carbon Equivalent

Mechanical properties are the core advantages of S690Q, particularly the combination of high yield strength and good toughness:

Impact toughness: For "Q" grade material, the impact test temperature is -20°C. If it is "S690QL," the impact temperature drops to -40°C ("L" stands for low-temperature toughness).

Thickness influence: When the plate thickness exceeds 16mm, a reasonable reduction in yield strength is allowed (e.g., ReH ≥ 650MPa for thicknesses of 50-100mm). Specific values should be referenced according to the standard or the steel mill’s quality assurance certificate.

EN 10025-6 S690Q is widely used in fields requiring heavy load-bearing, impact resistance, or lightweight design due to its combination of advantages, including "high strength, high toughness,  machinability, and weldability":

Construction Machinery: Key load-bearing structures such as booms, arms, and frames of excavators, cranes, and loaders.

Mining Machinery: Beds of mining dump trucks, crusher frames, and middle troughs of scraper conveyors.

Construction and Bridges: Load-bearing components of long-span steel bridges, steel skeletons of high-rise buildings, and tower crane bodies.

Energy Equipment: Flanges and connecting plates of wind turbine towers, as well as load-bearing components of oil and gas extraction platforms.

Special Vehicles: Frames of heavy-duty trucks and armored plates of bulletproof vehicles (some high-toughness specifications).

Delivery Conditions

Must be in the quenched and tempered (QT) condition: The steel is first subjected to high-temperature quenching (rapid cooling) to obtain a martensitic structure, followed by medium-to-high temperature tempering to transform the structure into tempered sorbite. This process ultimately achieves a balance of "high strength + high toughness" (avoiding the brittleness of the quenched state).

Surface Condition: Typically "as-rolled" or "finished surface." Subsequent treatments such as shot blasting or painting can be applied as needed.

Weldability

S690Q is weldable, but it requires strictly controlled welding procedures due to its high carbon equivalent (CEV). 

Temperature Grade and Quality Grade Differences

The impact performance of the S690 series steel grades is distinguished by suffix letters:

S690Q: Impact energy ≥30J at -20°C (longitudinal), ≥27J (transverse)

S690QL: Impact energy ≥30J at -40°C (longitudinal), ≥27J (transverse)

S690QL1: Impact energy ≥30J at -60°C (longitudinal), ≥27J (transverse)

Precautions for Procurement and Use

Clearly Define Technical Requirements: During procurement, specify thickness, width, length, impact temperature (Q/-20°C or QL/-40°C), surface condition, and additional inspection requirements (such as non-destructive testing and Z-direction properties).

Welding Process Control: Preheating is required before welding (recommended preheating temperature: 80-150°C, depending on plate thickness) to avoid cold cracks. Stress relief heat treatment (e.g., 600-650°C tempering) can be performed after welding.

Tool Selection for Machining: Due to high strength, use carbide tools (e.g., WC-Co alloy), reduce cutting speed, and increase feed rate to minimize tool wear.

Verification of Quality Certificate: Ensure the steel mill provides a quality certificate compliant with EN 10204 Type 3.1, confirming compliance with chemical composition, mechanical properties, heat treatment status, and other indicators.

Summary

EN 10025-6 S690Q is a premium, ultra-high-strength structural steel offering an exceptional strength-to-weight ratio. Its use allows for lighter and more efficient designs but demands a high level of expertise and controlled procedures during fabrication, especially welding.