Test low carbon boron steel ( % ：0. 13 〜0. 21C

0. 000 5 〜0. 003 7B) and low carbon Nb-V microalloying boron steel ( % ：0. 18 〜0. 24C

0-0. 033Nb

0 〜0. 050V

0. 000 5 〜0. 002 2B) were melted respectively by laboratory 50 kg vacuum induction furnace and carbon tube furnace

and the tensile specimen and notched delayed fracture specimen were turned from φ13 mm rolled-forged rod. The delayed fracture resistance of test steels water-quenched at 950 T and tempered at 200℃ is presented by delayed fracture strength ratio R =σ

n

( delayed fracture strength) /

σ

c

( tensile strength). Results showed that the tensile strength of 0. 000 5% B steel microalloying with 0. 028% Nb and 0. 032% V was 1 480 MPa and its R value was 0. 665 which w

as higher than that of 40Cr steel and 35CrMo steel queched at 880 ℃

oil cold + tempered at 400 ℃ (both σ

c

were respectively 1 460 MPa and 1 530 MPa

R value 0. 583 and 0. 556).