The developed new steel 4Cr3. 75MoSiV for extrusion wheel （/% ： 0. 39C

0. 95Si

0. 35Mn

3. 75Cr

1. 25Mo

0. 99V

0.009 5N） and common steel 4Cr5MoSiV for extrusion wheel （/% ： 0. 42C

1. 20Si

0. 38Mn

5. 04Cr

1. 14Mo

0. 85V

0.035 ON） are melted by a 10 kg vacuum induction furnace

and forged to Φ18 mm bar for test

then 880 ℃ 1 h +720 °C. 1 h spheroidal annealing

108 0 ℃ 15 min oil quenching

550 ℃ 2 h +530 ℃ 2 h double tempering. The morphology

size

precipitates ratio and distribution of main carbide precipitates in both steels as well as the effect of p

recipitates on 500 times 600 °C -ambient continuous thermal fatigue behavior are studied. Results show that V

4

C

3

+ VC amount to 12% of total precipitates and there is 26% Cr

23

C

6

precipitate in common steel 4Cr5MoSiV containing 5. 04% Cr

while in developed new steel 4Cr3. 75MoSiV the V

4

C

3

+ VC

which are favourable to increase thermal fatigue behavior

amount to 31% of total precipitates and the Cr

23

C

6

precipitate has not been found in new steel ； after continuous thermal fatigue test the surface crack of steel 4Cr5MoSiV with thermal fatigue factor 3. 18 x 10

-3

is coarser and larger than that of steel 4Cr3.75MoSiV with thermal fatigue factor 1. 77 x 10

-3

.