Structural steel
Application Area
After normalizing or cold forming, structural steels are primarily used in high-rise, civil, bridge, water, container, vehicle, and mechanical engineering due to their tensile strength and yield strength.
Description
Unalloyed structural steels make up the largest share of total steel production. They are typically supplied in a hot-rolled condition.
Properties
Welding: Structural steels are highly weldable.
Mechanical Properties: In condition +N, structural steels have high yield strength combined with sufficient plastic deformability, especially under impact loads. It is important to consider the temperature at which notched impact energy is tested.
| Designation (27J) | RJ | J0 | J2 | J3 | J4 | J5 | J6 |
|---|---|---|---|---|---|---|---|
| Test Temperature °C | +20 | 0 | -20 | -30 | -40 | -50 | -60 |
Technological Properties
This section describes properties such as machinability, cold formability (e.g., stamping, deep drawing for sheets), weldability, corrosion resistance, etc.
Physical Properties
Requirements related to magnetic properties, thermal conductivity, and thermal expansion.
Weldability
Due to the significant role of welding as a joining process in structural steels, the following explanations clarify the concept of weldability:
- Welding suitability: A steel is considered weldable if, based on its metallurgical, chemical, and physical properties, a weld can be produced that meets the required specifications.
- Welding safety:This is not determined by the steel manufacturer but by the fabricator. Welding safety is ensured when the welded component remains safe in operation (brittle fracture and crack-free) under the intended service conditions. It is influenced by Design aspects (plate thickness, weld type and arrangement, notch effect), Load conditions (type, magnitude, and multi-axiality of stresses, strain rate, operating temperature).
- Welding feasibility: It indicates whether the respective joint can be produced under the selected manufacturing conditions. The three influencing factors interact with each other.A steel is considered weldable if, during rapid cooling, the microstructure formed in the heat-affected zone (HAZ) remains sufficiently deformable. To achieve this, martensite formation must be avoided, or the carbon content must be limited so that any resulting martensite remains sufficiently ductile.In addition to carbon, alloying elements also influence the crack susceptibility of the hardened zones in the HAZ. The combined effect of carbon and alloying elements is often expressed using the carbon equivalent (Ceq). A commonly used formula for structural steels is: Ceq = C + Mn/6 + (Cr + Mo + V)/5 + (Ni+Cu)/15 [%]. Alloying elements in this equation are expressed as mass percentages. Compared to carbon, they contribute less significantly to crack susceptibility.
The alloying elements in this equation are expressed as mass percentages. Compared to carbon, they contribute significantly less to crack susceptibility. In general, depending on the workpiece thickness, steel with Ceq ≥ 0.45% is preheated to 100°C – 150°C before welding. This reduces the cooling rate in the heat-affected zone (HAZ), which effectively decreases the amount of martensite. As a result, the hardness in the HAZ is reduced, minimizing the risk of cracking.