Serious white spots defect occurs on the surface of (4.4～4.6) mm×2050 mm wide-thick cold rolled strip of 316L stainless steel (/%:0.02C

17. 12Cr

10. 15Ni

2.06Mo) for tank container in production process. The causes of the white spots are researched by means of feature analysis

steels grades comparison and on-site defect tracking

the results show that there is much emulsion on the surface of cold rolled strip

on which the formed oxide scale is unevenly distributed in case of incompletely degreasing and annealing at 980 ℃

the color of the oxide scale on the surface with much emulsion is white

these white oxide scales are changed into white spots after pickling. Besides

the influencing factors of the residu al emulsion on the surface of rolled strip and the removal effect of residual emulsion are systematically studied. By applying oil scraping system for nonwoven-fabric roller

adding air knife purging system

controlling emulsion concentration 1.5%~ 3% and temperature(55±3)℃

repairing two sets of magnetic filtration system in rolling process

through controlling de- greasing electrical conductivity7.5 μs/cm and loop filling speed 50 m/min

adjusting9～12 area Furnace temperature to 1030 ℃

the unqualified rate of white spots is reduced from 10% to 0.5% or less.