The right choice of steel – according to the Eurocode - Economical Bridge Solutions based on Innovative Composite Dowels and Integrated Abutments - page 12

Simplified aids are given to choose the appropriate material with sufficient fracture
toughness to avoid brittle fracture.
Through-thickness properties.
Guidance on the choice of through-thickness properties is given in EN 1993-1-10 (2005).

With reference to these requirements, the designation of the steel grade is defined in the
product standard for hot-rolled products and structural steels in EN 10025 (2004) (“Fig. 3”).
The classification of steel grades is accordingly based on the minimum specified yield
strength at ambient temperature.

Figure 3: Designation of the steel grade according to EN 10025 (2004)

The product standard also differentiates the delivery condition. These are defined by the
rolling process of the steel. Conventional hot rolling takes place in the recrystallised austenitic
phase (g) and is followed by a subsequent air cooling, see Fig. 4. Without any special rolling
control or heat treatment, this material delivery condition is specified to be “as rolled” (AR) in
the EN 10025. With an additional normalising thermal treatment (N), the steel microstructure
can be refined leading to improved properties specifically described in EN 10025-3, if alloying
elements have been added to the steel. The reduction of the grain size leads to an increase of
the specific surface of the grain boundaries within the material. Since these grain boundaries
represent an obstacle to deformation the yield strength increases. The fine grained structure of
normalised steels may further be improved with restriction of the alloying elements via a ther-
momechanical treatment. This may be quenching (Q) with water or oil, an accelerated cooling
to about 500°C after rolling and lastly slower cooling to the room temperature, and a succes-
sive tempering (T) to regain ductility. A fine grained microstructure may also be obtained if the
hot rolling process is carried out with a control of the temperature during the final deformation
(CR). Another possibility to refine the microstructure is to apply a thermomechanical rolling
process according to EN 10025-4. Hereby, rolling is also performed with a controlled rolling
process in the recrystallised austenitic and further rollings in the non-crystallised austenitic
phase, in cases even into the austenitic-ferritic phases (α + g) (M/N). For thermomechanical
steels (TM), rolling is carried out at lower temperatures than normalising rolling; the rolling