The production flow sheet for developed 20 mm low-cost niobium-titanium-boron microalloying low carbon steel plate (/% : 0. 06C
0. 40Si
1. 60Mn
0. 010P
0. 005S
0. 050Nb
0. 012Ti
0. 002B) is 130 t top and bottom combined blown converter-LF-RH-250 mm slab casting-4300 mill rolling-direct quenching-tempering process. With end rolling ≥ 900 °C
>20 °C./s direct quenching and tempering at 500 ℃ the tensile strength R
m
of the 20 mm steel plate is 855 MPa
yield strength R
p
0.2771 MPa
elongation A 16% impact energy A
KV2
at 0 °C 217 〜238 J and impact energy 4KV2 at -40
°C
137 〜181 J. Tempered structure of the steel is fine bainite lath with width 0. 5 ~ 1. 0μm and there are lots of dispersed 30 ~90 nm Nb + Ti carbo-nitride precipitates.