US 2006/0037868 A1Published application

Process and plant for descaling, pickling and finishing passivating stainless steel strips, and strips so obtainable

AssigneesCentro Sviluppo Materiali S.p.A., Thyssenkrupp Acciai Speciali Terni S.p.A., Henkel AG & Co. KGaA

InventorsAlessandro Dulcetti, Stefano Luperi, Gianvincenzo Salamone, Giovanni Vespasiani, Stefano Mantovani, Yean Demertzis

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Abstract

Environment-friendly process for descaling, pickling and finishing/passivating in a continuous, integrated and flexible manner, applicable to any type of stainless steel, regardless of its physical structure, chemical composition and nature of surface oxides to be removed, within a single plant in which the required chemical, electrochemical, mechanical and hydromechanical treatments are carried out, merely diversifying the operative conditions of each treatment according to the stainless steel type to be processed. The process and the plant according to the invention allow elevated reaction rate, excellent surface quality, low energy and chemical reagent consumption and total environmental compatibility. The figure shows the block diagram of an embodiment of the plant according to the invention.

A process for descaling, pickling and finishing/passivating stainless steel strips, wherein the strip to be processed is subjected to the following sequence of steps:

an electrochemical and/or chemical descaling treatment, differentiated into two hydraulically distinct units using aqueous solutions, wherein:
said descaling, in case of electrolytic treatment, uses aqueous solutions containing in the first unit:
from 10 to 250 g/l H2SO4;
with <80 g/l total dissolved Fe;
and optionally
≧15 g/l Fe+3, with Fe+3/Fe+2>1.0;
and, in the second unit:
from 10 to 250 g/l H2SO4;
≧2 g/l Fe3+;
with <80 g/l total dissolved Fe;
and optionally
with Fe3+/Fe2+≧1.0;
with induction onto the strip of at least one anode-cathode or anode-cathode-anode polarity sequence applied by pairs of electrodes having the same polarity, between which the strip runs, with anodic treatment times (ta) and anodic current density (I) selected so as to satisfy the relation
ta>k+c/I
where:
ta is the anodic treatment time [s];
k is an experimental constant, its values ranging from 2 to 15 s;
c is an experimental constant, its values ranging from 40 to 120 C/dm2;
and
I is the anodic current density, its values ranging from 1 to 100 a/dm2
and with times during which the strip is not subjected to the electric field that range from the 5% to the 60% of the total electrolytic descaling time;
said descaling, in case of chemical treatm ent, uses aqueous solutions containing in both units
from 25 to 280 g/l H2SO4;
said strip being subjected to said descaling treatment for an overall time comprised in the range 10-250 s, at temperatures comprised in the range 20-105° C.;
a chemical pickling, optional, and/or finishing/passivating treatment, differentiated into two hydraulically distinct units, wherein:
said chemical pickling treatment uses aqueous solutions containing:
from 20 to 180 g/l H2SO4;
from 5 to 50 g/l free HF;
≧15 g/l Fe+3;
Fe+3/Fe+2>0.8;
with <80 g/l total dissolved Fe;
said chemical finishing/passivating treatment uses aqueous solutions containing:
from 10 to 100 g/l H2SO4;
from 0 to 15 g/l Free HF;
<20 g/l Fe+3;
>0.03 mol/l H2O2;
said strip being subjected to said chemical pickling and/or finishing/passivating treatment for an overall time ranging from 2 to 250 s, at temperatures ranging from 20 to 80° C.;
and
there being provided, for each pickling and/or finishing/passivating treatment unit, a solution recirculation with a flow rate equal to at least 10 dm3/h per m2 of strip;
there being provided, at least at the end of each descaling, pickling and/or finishing/passivating treatment unit, a mechanical and/or hydromechanical and/or water-employing cleaning treatment, optionally by water jets having a >100 bar pressure.

37 more elements

Abstract

Claims19