The depressed defects of scales with dimension (10~40) mm x (3~10) mm and depth 1~3 mm occur at 30 mm plate hot-rolled from 230 mm casting slab of steel Q235B (/% : 0.18C

0.20Si

0.34Mn

0.015P

0.005S

0.07Cr). The primary scale formation mechanism and removability at high temperature in high-pressure water descaling operation are analyzed and the measures to optimize the process are forward including (1) with optimizing heating process to insure the casting slab surface temperature being more than 1170 °C for high-pressure water descaling； (2) with optimized high-pressure water parameters i. e. increasing spray-water pressure from 18 MPa to 20 MPa and flow velocity from 161.2 m/s to 170.0 m/s

in order to increasing the impact force from 116.3 N to 122.7 N. The test and commercial production results by 26 heats show that after process optimization the closing off rate of slab caused by rolled-in scales reduces to less than 2%

it is obvious to improve the rolled-in scales defects.