Material for test of hot rolled steel sheet pile SY390BZ (% ： 0. 23C

0.44Si

0.91Mn

0.06V

0.010P 0. 002S) is melted by a 55 kg vacuum induction furnace

cast to Φ170 mm round ingot

rolled to 12 mm sheet with 20% reduction in last flat pass and finishing rolling at 880 〜900 °C

air cooling. The tough-brittle transition temperature of test steel is measured and the morphology of low temperature impact fracture and structure of steel at 20 °C to - 60 °C is observed by using scanning electro-microscope Quanta 400 and optical microscope Olympus GX71. Results show that there are a lot of large equiaxial dimples in fracture at ambient temperature and the steel has higher impact energy (130 J)

while there are smaller shallow dimples in fracture at -40 °C and the impact energy of steel decreases to 30 J. The tough-brittle transition temperature of the steel

calculated by using energy calculation method is -30 °C。 The banded structure with A1

2

O

3

inclusion occurs in steel led to lower impact toughness.