Microstructure and properties of Nb-Ti microalloyed ultra-high-strength steel (/% ： 0. 36C

0. 45Si

0. 63Mn

0.005P

0.002S

0. 83Ni

1.2Cr

0. 23Mo

0.04Nb

0.01 Ti

0.004 0N

0.002 00) are studied by tensile tester

impact tester

metallographic microscope

transmission electron microscopy and other equipments. Of 200 kg VIM furnace melting with initial rolling at 1 050 °C and finishing rolled at 850 °C

the test steel organization mainly structure is bainite and pearlite. After quenching at 890 °C and tempering at 210 °C

the mechanical properties of steel are R

m

1 906 MPa

R

p0.2

1574 MPa

A 10. 0% and -40 °C 18 J

t

he organization has a small amount of lath martensite with both ends of a square

in addition to a mass of acicular martensite with both ends of sharp-angled y mostly with grain boundary angle in 60°

and its substructure appears a large number of twin

accompanied by microalloyed precipitates ； the microalloyed precipitates are mainly round or oval small size NbC. And a small amount of square and circular larger size (Ti

Nb) (C

N) precipitates

the greater size

the more obvious square characteristics

and the niobium content of precipitate phase reduces

the titanium content increases.