The process flowsheet of 1. 2 mm cold-rolled sheet of steel DC04-LQQ (/% : ≤0.003 5C

≤ 0.02Si

0. 08 ~0. 20Mn

≤ 0. 015P

≤ 0. 012S

0. 02 ~ 0. 06Als

0. 06 ~ 0. 08Ti

≤0.0040N) is hot metal pretreatment-250 t BOF-RH-slab casting-hot-rolling- cold-rolling- annealing at 820 -840 °C. The analysis on surface defects of deep-drawing piece of steel DC04-LQQ for filter has been carried out； the results show that the main surface defects are tongue-shaped and pinhole-shaped defects

the tongue-shaped surface defects containing O

Al

Mg and Fe etc elements are formed by Al

2

O

3

and MgO inclusions in liquid and the pinhole-shaped surface defects containing K

Na

Ca

Al and Ti etc elements are form

ed by entrapped mold powder in condition of large fluctuation of mold liquid surface. With the measures including increasing holding time of liquid from original 20 min to 30 min

extending the interval between adding Al deoxidizing and adding alloys from 1 ~2 min to 4 min in RH refining

optimizing tundish argon blowing scheme and revamping the tundish cover argon pipeline

and increasing the tundish argon pipe diameter from original 20 mm to 34 mm

the occurring rate of surface defects of steel for filter decreases from original 7. 78% to 2. 62% to get obvious effect.