The coils were undergone the additional controlled air cooling to 450°C, which
was carried out using industrial aerators [25, 26]. After this technological operation,
the coils were taken to the warehouse for final ambient cooling.
The technology of additional controlled air cooling of the rolled products
following the thermomechanical rolling is specially developed for the production of
this pilot batch. The deployment of this technology made it possible to reduce the
thickness of air scale, which is intensively built up once the rolling is finished and up
to 450°C, with typical air cooling.
Analysis and discussion of the results. The quality analysis of the obtained
results was carried out based on the mechanical tests and metallographic research.
The samples for evaluation of mechanical properties from coils of the pilot batch
were taken after the final cooling. The mechanical testing results are shown in Table 6.
The results of the mechanical tests are fully in line with the requirements
described in EN 10149-2 for steel grade S355MC. It should be said that the test results
obtained are at the upper level of the standard requirements. Thus, the production
technology can be adjusted towards decreasing microalloyed elements, thus reducing
the product cost. Also, the results obtained show the technical potential for further
development of the technology of thermomechanical rolling of steel grade S355MC in
larger thicknesses, and the development of hot rolling coil production of higher
strength categories.
Table 6 Mechanical testing results for coil pilot batch
Heat No.
Mechanical properties
Yield strength,
MPa
Tensile
strength, MPa
Elongation,
%
Mandrel bending
255634-2
475
550
30
Without cracking
EN10149-2 requirements
for S355MC
≥355
430-550
23
Without cracking
For the analysis of internal quality of the rolled products, metallographic studies
of the samples from coils (heat 255634-2) were carried out. The microstructure of the
samples is shown in Fig. 4.
surface
axis
surface
Figure 4. Microstructure of samples (heat 255634-2) after etching, × 200
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