S - stands for structural steel
355 - stands for minimum yield strength (though it will vary based on thickness, refer below section for more details)
Normally S355 will have following variation based on impact strength at different temperatures, as steel become more brittle when temperature drops. If your structure need to withstand -20°C, it's better and safer to choose S355J2.
S355JR - Can withstand an impact energy of 27J at +20°C
S355J0 - Can withstand an impact energy of 27J at 0°C
S355J2 - Can withstand an impact energy of 27J at -20°C
S355K2 - Can withstand an impact energy of 27J at -30°C (40J at -20°C)
S355 is named based on it's minimum yield strength of 355 Mpa (N/mm2), However the yield strength reduces when you go up in thickness above 16 mm for flat products & hollow sections
Here is the table showing reduction in yield strength based on thickness
Up to 16 mm - 355 Mpa
16 mm < t ≤ 40 mm - 345 Mpa
40 mm < t ≤ 63 mm - 335 Mpa
63 mm < t ≤ 80 mm - 325 Mpa
80 mm < t ≤ 100 mm - 315 Mpa
100 mm < t ≤ 150 mm - 295 Mpa
S355 Tensile Strength
S355 ultimate tensile strength ranges also varies based on thickness.
Up to 3 mm - 510 Mpa to 680 Mpa
3 mm < t ≤ 100 mm - 470 Mpa to 630 Mpa
100 mm < t ≤ 150 mm - 450 Mpa to 600 Mpa
