Developed Nb-Ti microalloying high strength steel (/% : 0. 04C

0. 34Si

1.40Mn

0. 010P

0. 004S

0. 098Nb

0. 020Ti

0. 045Al

0.002 5N) is melted by a vacuum induction furnace with casting 50kg ingot

forged to 40 mm plate

and rolled at 1 200 T to 10 mm plate by lab single strand mill with 7 pass

finishing pass reduction ratio ≥ 15%

finishing temperature 880 °C

splashing water cooling to 600 °C

and in furnace at 600 T for 30 min

furnace cooling to ambient temperature to simulate the laminar flow cooling and coil process. With test by Gleeble 3500 thermal simulation machine

the low plasticity zones of steel at high temperature are 650 〜800 °C and ≥ 1 300 ℃ . Mechanical properties examination results are lower yield strength R

el

625 ~640 MPa

tensile strength R

m

705 〜710 MPa

and elongation 18. 0% 〜 19. 5%

. The developed steel has the characteristics such as low carbon equivalent

nice weldability

and low cost.