The growth behavior of austenite grain of non-quenching-tempering steel SG4201 for direct turning-cutting (/% : 0.42C

0.50Si

1.40Mn

0.009P

0.005S

0.02Nb

0.06V

0.015N) heating at 1000-1250 T for 0-300 s has been tested and studied by using Gleeble 3500 thermal simulation test machine

and the growth model of austenite grain of steel has been established. Test results show that with heating for 30 s

the coarsening temperature of austenite grains with large amount niobium solving is about 1100 ℃； the activation energy of austenite grain growth is about 110.8 kJ； and the soaking temperature of steel SG4201 casting billet being no-more than 1150 °C is defined. The commercial production results show that with casting billet soaking heating at ≤1150℃

finished rolled at 800-850℃

cooling speed after rolling 30-35℃/s

the mechanical properties of steel SG4201 are tensile strength 927 MPa

yield strength 687 MPa

elongation 23.5%

reduction of area 57%

U-impact energy 48 J and HBW hardness value 265.