HSLA Steel Q420N ( /% ：0. 16C

0. 28Si

1.39Mn

0. 015P

0. 003S

0. 11Cr

0. 009N ) at production line is 120 t BOF-LF-RH-300 mm × 340 mm casting bloom-hot continuous rolling to Φ90 mm bar product. The effect of hot- rolled billet rolling to bar product processes by normal rolling ( beginning rolling at 1 100 ~ 1 150 ℃

finishing rolling at 950 ~1 000 ℃

to cooling bed at 900 ~950 ℃，cooling rate 0. 5 ~1.0 ℃/s ) and controlled rolling and controlled cooling (beginning rolling at 1 100 ~ 1 150 ℃

finishing rolling at 800 ~850 ℃

to cooling bed at 700 ~750 ℃

cooling rate 5. 0 ~15 ℃/s ) on microstructure and mechanical properties of steel Q420N is tested and studied. The results show that the microstructure of Q420N steel is improved and yield strength is increased obviously with decreasing the finishing temperature and increasing the cooling rate. The yield strength of Q420N steel by normal rolling process is 325 ~340 MPa and the grain size rating is 6. 0

while the yield strength of Q420N steel by controlled rolling and controlled cooling process is 495 ~ 515 MPa and the grain size rating is 9. 0 which meets the standard requirements and reduces production cost.