The production flowsheet for V-Nb-Ti microalloying S355NL large size seamless tube for low-temperature structure (/% ： 0.14~0.15C

0.29~0.30Si

1.35〜1.37Mn

≤0.014P

≤0.005S

0.02~0.03Nb

0.06~0.07V

0.010~0.020Ti

0.39~0.41 C

eq)

is 100t EAF-LF-VD-feeding Ca wire-φ400 mm round bloom casting- 3-roll mill trainnormalizing treatment at 940 °C for 30 min

air cooling. The statistic results of 16 heats and batches show that with the process measures including adding silicon -manganese and fenrrosilico-aluminium alloy in tapping of EAF

using CaO-SiO

2

- Al

2

O

3

slag series and whole process argon blowing in LF refining

VD treating for 20~25 min

using long nozz

le and argon blowing in casting process

the object quality and properties of steel tube are constant with uniform structure and fine grains

and the each property index meets or exceeds the technical requirement of standard and the purchaser

i. e. yield strength R

eH

380~435 MPa

tensile strength R

m

522~605 MPa

elongation A 24%~29% and impact energy at -50℃ 132~237 J.