Patent Application
20060037868
The present invention refers to the descaling, the pickling and the finishing/passivating, in the absence of nitric acid and of molten salt baths, for stainless steel strips, previously hot-rolled and optionally annealed or cold-rolled and annealed.
As it is known, stainless steel pickling is carried out in order to eliminate the scale of thermal oxides generated during hot rolling and/or annealing treatments, and to dissolve the chromium-depleted alloy layer (dechromized layer) therebelow. This process is conventionally carried out in three distinct and separate process steps: a first step of descaling, i.e. of chemical-physical modification of the scale, with partial or total removal of the latter; a second step of actual pickling, i.e. of removing any residual scale and the underlying chromium-depleted alloy layer; and a third step, of surface finishing and passivating. In several instances, the last two steps of pickling and of finishing/passivating may be carried out jointly.
The state of the art envisages several manners for conducting the descaling step, depending on the type of oxide present onto the metal at the end of the metallurgical treatments.
In order to remove the oxide generated in the hot rolling and annealing processes, the descaling, pickling and finishing/passivating treatments are usually preceded by scale-breaking treatments (scale-breaker, grit-blasting and abrasive brushing) which fragment and partially remove said scale. For cold-rolled stainless steel products the scale is not mechanically preconditioned, as often this treatment is not compatible with the surface quality required for the finished product.
Generally, for the step of descaling cold-rolled and annealed stainless steel processes are resorted to capable of inducing a substantial modification of the oxides, facilitating the dissolving thereof.
To this end, the most widely adopted methodologies are:
a) thermo-chemical descaling, consisting in the immersing the material to be pickled in an oxidant molten salt bath (400-600° C.) capable of altering the scale by increasing the oxidation grade of the oxide-constituting metals. In particular, Kolene (eutectic of the ternary system NaOH, NaNO3 and NaCl) baths at temperatures of about 500° C. are widely resorted to;
b) electrolytic descaling by neutral solutions of sulfates or acid solutions, with partial modification of the oxidation states of the metals constituting the scale and the resulting dissolution of the latter.
Generally, the stainless steel pickling step is conducted using acid baths having an elevated oxidizing ability, capable of dissolving the underlying chromium-depleted alloy layer.
These baths mainly consist of mineral acid mixtures, the most widespread thereamong being:
1) nitric and hydrofluoric acid mixtures at temperatures generally ranging from 50 to 75° C.;
2) sulfuric, hydrofluoric, hydrochloric and phosphoric acid mixtures with additions of highly oxidizing agents, among which, e.g., permanganates, persulfates, ferric chloride, ferric sulfate and hydrogen peroxide, with a temperature ranging from 50 to 100° C.
The passivating/finishing step is also aimed at generating a protective passive film. When not carried out in the same pickling step, it is usually attained in baths having a high redox potential. These baths mainly contain nitric acid or the abovementioned acids and oxidants in lesser concentrations and with a lesser content of steel-constituting metal ions.
Moreover, to date several processes do exist related to stainless steel descaling and pickling steps, based on the employ of nitric acid-free acid solutions are available. In particular, stainless steel pickling processes are known based on the use of nitric acid-free acid solutions, whose oxidizing power derives from the presence of different elements, among which ferric ions, hydrogen peroxide and persulfates.
In particular, DE-A-19624436, WO 9826111, EP-A-763609 and JP95-130582 describe processes of descaling as well as of pickling in a nitric acid-free acid solution, also with the use of DC-powered electrolytic cells (current density ranging from 0.5 to 250 A/dm2).
DE-C-3937438 describes a process in which the direct electric current is employed for the reoxidation of ferrous ions to ferric ions in a hydrochloric acid solution.
EP-A-838542 describes a descaling process in an aqueous solution of sodium sulfate, with a concentration ranging from 10 to 350 g/l, wherein the strip is vertically passed through pairs of counterelectrodes between which direct electric current having a density ranging from 20 to 250 A/dm2 is applied.
EP0582121 and EP0505606 describe chemical pickling processes wherein the strip is immersed in sulfuric acid-based solutions containing ferric ions and hydrofluoric acid, in which ferrous ion reoxidation is mainly carried out by addition of hydrogen peroxide under continuous air injection.
However, the known technologies summarily reported hereinbefore present significant drawbacks of environmental nature and of working safety, as well as of pickling process management in terms of control and costs.
Chemical descaling carried out with molten salts is particularly difficult to manage, due to the dangerousness associated to the elevated temperature (400-600° C.) of the bath, as well as to the difficulty of treating the rinsing solutions of the metallic product to be descaled. In fact, these rinsing solutions contain non-negligible quantities of toxic hexavalent chromium and of nitrites and nitrates. In neutral sodium sulfate solutions as well the electrolytic descaling exhibit the same drawback: in fact, the process fluids contain non-negligible quantities of hexavalent chromium. Electrolytic descaling treatments in sulfuric acid-based solutions, though overcome the drawback of chromium (VI), can determine an inferior surface quality by effect of the localized attacks caused by the sulfuric acid in sections not subjected to the electric field.
The employ of baths containing nitric acid for the pickling and the finishing/passivating steps entails relevant environmental problems, due to several reasons. Among the latter, the most important are as follows:
A) difficulty of safely abating the highly polluting nitrogen oxides (NOx) developed from the acid-metal reactions;
B) difficulty of respecting the environmental provisions in force for the disposal of spent solutions with regard to their elevated nitrate content;
C) utmost difficulty of keeping nitrate concentrations within the limits set by the provisions in force for said species in rinsing waters.
The present invention allows to overcome all of the abovementioned drawbacks, with the further advantage of providing an environment-friendly process for descaling, pickling and finishing/passivating in a continuous, integrated and flexible manner which is applicable to any type of stainless steel strip within a single plant by adopting a sequence of treatments, wherein the operative conditions of each treatment are diversified according to the typology of stainless steel to be processed as well as to the nature of the oxides to be removed present onto the stainless steel strip surface.
In fact, a subject of the present invention is 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:
In the electrolytic descaling treatments, the application of anode-cathode or anode-cathode-anode polarity sequences onto the strip may be attained connecting the electrodes to the power supply units ‘per face’, i.e. connecting each power supply unit exclusively to electrodes adjacent and facing a same face of the strip. The anodic/cathodic polarization time ratio of the strip is ≧1.5.
For the electrolytic descaling of austenitic stainless steel strips, in the two electrolytic units can be used the following solutions:
For ferritic or martensitic stainless steel strips, at least in the descaling treatment, can be used solutions distinct for composition and/or temperatures, having a >20 g/l ferric (Fe+3) ion concentration and a Fe+3/Fe+2 concentration ratio of >1.5.
Moreover, for the electrolytic descaling treatment of ferritic or martensitic stainless steel strips, the time during which the strip is not subjected to the electric field ranges from the 15% to the 25% of the total electrolytic descaling time.
For optionally annealed hot-rolled stainless steel strips, the descaling treatment may optionally be preceded by a mechanical and/or hydromechanical scale breaking/removing treatment.
The chemical pickling treatment for austenitic stainless steels may be carried out in aqueous solutions optionally distinct in composition and/or temperatures and consisting of:
For optionally annealed hot-rolled ferritic and martensitic stainless steels the chemical pickling and finishing/passivating treatments may be carried out, respectively:
as to the pickling treatment, in aqueous solutions containing
from 40 to 180 g/l H2SO4;
from 5 to 50 g/l free HF;
≧20 Fe3+, with Fe3+/Fe2+>0.8;
<80 g/l dissolved Fe;
at temperatures ranging from 20 to 70° C.;
for times ranging from 10 to 160 s;
and, as to the finishing treatment, in aqueous solutions containing;
from 20 to 100 g/l H2SO4;
from 0 to 35 g/l free HF;
<20 g/l Fe3+;
>0.03 mol/l H2O2;
at temperatures ranging from 20 to 50° C.;
for times ranging from 2 to 80 s.
The ferritic and martensitic steels, cold rolled and annealed after the electrolytic descaling treatment, may be subjected to the sole finishing treatment in aqueous solutions containing:
from 20 to 70 g/l H2SO4;
<15 g/l Fe3+;
>0.05 mol/l H2O2;
from 0 to 15 g/l free HF;
at temperatures ranging from 20 to 50° C.;
for times ranging from 5 to 80 s.
Preferably, the finishing/passivating treatment is carried out applying the solution with spraying nozzles allowing a turbulent and homogeneous distribution of the solution onto the surfaces of the strip to be treated, so as to ensure a flow rate not lower than 15 dm3/h per m2 of strip.
In order to assure effectiveness of the end treatment and high quality of the end product, the surfaces of the strips, which after the descaling treatment are subjected to the sole finishing/passivating treatment, in the time interval between the former and the latter are kept moist in a room saturated with ambient-pressure steam.
In order to maintain the correct concentration of the ferric ions Fe+3 and Fe+3/Fe+2 concentration ratio, an at least stoichiometrical quantity of optionally stabilized hydrogen peroxide is inletted in the descaling and pickling solutions.
While crossing the chemical pickling units, the strips, subjected to the sole finishing/passivating treatment after the descaling treatment, are kept with their surfaces moist, in a room saturated with ambient-pressure steam.
The invention also refers to the descaled, pickled and finished/passivated stainless steel strips obtainable with the abovedescribed process.
A further subject of the invention is a plant suitable for carrying out the environment-friendly process for descaling, pickling and finishing/passivating, in a continuous, integrated and flexible manner, stainless steel strips as abovedescribed.
The plant suitable for carrying out the process according to the invention comprises:
The ‘per face’ connection of the electrodes to the power supply units (i.e. each power supply unit being exclusively connected to adjacent electrodes of a same face of the strip) makes the current lines outletted from each single power supply unit interest only one side of the strip, regardless of the position of the latter, ensuring homogeneity of treatment, in terms of current density, onto both faces of the strip. Moreover, the ratio between the lengths of the electrodes inducing the anodic and the cathodic polarity is greater than 1.5 and the time during which the strip is not subjected to the electric field ranges from the 5% to the 60%, preferably from the 15% to the 30%, of the total electrolytic descaling time.
According to the present invention, the polarity sequence is such that the strip outletted from the electrolytic sections is subjected to anodic polarization as a last step, in order to facilitate the generation of a protective passive film.
The means for controlling the temperatures of the solutions in the different units may be heat exchangers.
The means for spraying the finishing/passivating solution onto the strip may be spraying nozzles capable of ensuring a flow rate at least equal to 15 dm3/h per m2 of strip and a turbulence at the strip/solution interface having, at quiescent line and onto the top face of the strip, a Reynolds number equal to at least 50,000.
Apart from the advantageous availability of an integrated and flexible system, the process and the plant for descaling, pickling and finishing/passivating stainless steel strips according to the present invention exhibit the following advantages:
So far, the invention has been generally outlined. With the aid of the annexed figures and of the following examples, hereinafter a more detailed description of embodiments thereof will be given, aimed at making apparent the objects, the advantages and the operation modes thereof.
In this example there are described the representative sections of a typical plant capable of treating hot-rolled optionally annealed stainless steel strip and cold-rolled and annealed stainless steel strips having a width ranging from 900 to 1,600 mm and a thickness ranging from 0.3 to 3 mm at rates ranging from 10 to 100 m/min, consisting of:
Moreover, for each unit there have been installed bath controlling and managing devices, allowing analyzing, automated reagent additioning and spent solution scavenging.
Electrolytic/Chemical Descaling Treatment Section
This section consists of two hydraulically distinct descaling units, the first unit comprising four electrolytic cells and the second unit comprising two electrolytic cells, each of a length equal to about 8 m. Each cell is provided with three pairs of electrodes facing the strip, electrically separated thereamong and located so as to realize a cathode-anode-cathode sequence on the strip. Each electrolytic cell is equipped with two DC power supply unit, each of the latter being capable of outputting a maximum direct current equal to 7.5 kA. Each power supply unit is connected to three electrodes located onto the same face with respect to the strip surface.
The overall length of the pairs of electrodes anodically polarizing the strip is equal to 21.6 m, satisfying the relation L>(c/I+k)v where the values of I, v, c and k are respectively equal to 12 A/dm2, 1.67 m/s, 90 C/dm2 and 4 s. The electrode width is equal to about 1.8 m.
In
At the outlet of the first descaling unit there is a hydromechanical cleaning system consisting of a water jet rinsing, as well as of a pair of brush rolls acting onto the two faces of the strip for removing detached but not yet removed oxide particles. At the outlet of the second descaling unit there is a second hydromechanical cleaning system consisting of a water jet rinsing and of a pair of brush rolls, upstream of a high-pressure rinsing system capable of sending onto the two faces of the strip a rinsing water flow equal to at least 20 m3/h at a pressure of about 120 bar. The temperature of the descaling solution is held at the desired values with heat exchangers and steam jets internal to the different treatment units. The system for controlling the temperature of the descaling solutions allows to set and to hold temperatures ranging from 40 to 80° C.
Chemical Pickling and Finishing/Passivating Treatment Section
The chemical treatment section consists of two hydraulically distinct units, the first one dedicated to the chemical pickling and the second one dedicated to the surface finishing/passivating treatment. At the outlet of the second unit there is installed a hydromechanical cleaning system consisting of a water jet rinsing and of a pair of brush rolls, upstream of a high-pressure rinsing system capable of sending onto the two faces of the strip a rinsing water flow equal to at least 20 m3/h at a pressure of about 120 bar.
The pickling unit consists of two 2 tanks having a length equal to about 18 m, the strip being immersed in the bath in each one thereof, both having a system for recirculating the pickling solution with an overall flow rate of about 400 m3/h. Said tanks provide an elevated turbulence at the solution-strip interface, with a Reynolds number value at least equal to 10.000 at quiescent line. The working temperatures are attained via steam flows, whereas temperature control is attained with a system of heat exchangers made of a material resistant to the HF-containing oxidizing pickling solutions.
For strips which after the descaling process should be subjected to the sole finishing/passivating treatment, inside of the two pickling unit tanks there are installed moisturizing systems that are capable, in the absence of pickling solutions, of keeping moist both strip surfaces in a room saturated with ambient-pressure steam. The finishing/passivating unit is realized with a single tank having a length equal to 21 m, inside which the strip is subjected to the action of the finishing/passivating solution by a spraying system at a pressure of about 0.5 bar capable of ensuring a solution flow rate equal to about 300 m3/h and an elevated turbulence at the solution-strip interface, having onto the top surface of the strip a Reynolds number, at quiescent line, equal to about 60.000. The working temperatures are attained by steam flows, whereas the temperature control is ensured by the presence of a system of heat exchangers made of materials resistant to HF-containing oxidizing pickling solutions.
In this example there is described the process for descaling, pickling and finishing/passivating a cold-rolled and annealed AISI 409 type ferritic stainless steel coil having 0.8 mm thickness, 1,270 mm width, 19.6 t weight. According to the present invention, using the line described in example 1, operation was carried out under the following conditions: c=68 C/dm2, k=7.2 s and rate=80 m/min.
After the electrolytic descaling treatment, the strip is subjected to the finishing/passivating and cleaning treatments in order to generate a surface passive film.
During the crossing of the chemical pickling unit the strip surfaces were kept moist in a room saturated with ambient-pressure steam.
At the end of the finishing/passivating treatment, hence at the line outlet, the strip appears completely pickled and having a good surface aspect. Reflection measuring conducted with a reflectometer (angle of reflection=60°) yielded values ranging from 28 to 35%. Scanning electron microscope (SEM) investigations of the surface layer proved the absence of residual oxides thereon.
During strip processing, the automated systems for controlling bath concentrations duly kept the provided concentrations by adjusting fresh reagent flows and spent solution discharges.
In this example there is described, the treatment for descaling, pickling and passivating a cold-rolled and annealed AISI 430 type ferritic stainless steel coil having 0.5 mm thickness, 1,570 mm width, 20.4 t weight. According to the present invention, using the line described in example 1, operation was carried out under the following conditions: c=74 C/dm2, k=8 s and rate=90 m/min.
At the end of this treatment, the strip surface appears perfectly descaled and free from detached but yet not removed oxide residues.
After the electrolytic descaling treatment the strip is subjected to the subsequent finishing/passivating and cleaning treatments, in order to generate a surface passive film.
During the crossing of the chemical pickling unit the strip surfaces were kept moist in a room saturated with ambient-pressure steam.
At the outlet of this section, hence at the line outlet, the strip appears completely pickled and having a good surface aspect. Reflection measuring conducted with a reflectometer (angle of reflection=60°) yielded values ranging from 40 to 44%. SEM investigations of the surface layer proved the absence of residual oxides and of localized attacks thereon.
During strip processing, the automated systems for controlling bath concentrations duly kept the provided concentrations by adjusting fresh reagent flows and spent solution discharges.
In this example there is reported the descaling, pickling and finishing treatment of a cold-rolled and annealed AISI 304 type austenitic stainless steel coil having 1.2 mm thickness, 1,570 mm width, 18.5 t weight. According to the present invention, using the line described in example 1, operation was carried out under the following conditions: c=65 C/dm2, k=3 s and rate=75 m/min.
At the outlet of this section, the surface appears descaled and free from residual oxides. The visual appearance is not yet that of the finished product. After the electrolytic descaling treatment the strip is subjected to the subsequent chemical pickling and cleaning treatments in order to generate a surface passive film.
At the outlet of this section, hence at the line outlet, the strip appears completely pickled and having a good surface aspect. SEM investigations of the surface layer proved the absence of residual oxides and of attacks to grain boundaries. Electrochemical testing demonstrated the absence of residual chromium-depleted layers and the presence of a good-quality passive film.
During strip processing, the automated systems for controlling bath concentrations duly kept the provided concentrations by adjusting fresh reagent flows and spent solution discharges.
Herein it is exemplified the process for descaling, pickling and passivating a cold-rolled and annealed AISI 430 type ferritic stainless steel coil having 1.0 mm thickness, 1,020 mm width, 16.6 t weight.
According to the present invention, using the line described in example 1, operation was carried out under the following conditions: c=68 C/dm2, k=7.1 s and rate=90 m/min.
After the electrolytic descaling treatment, the strip is subjected to the subsequent finishing/passivating and cleaning treatments in order to generate a surface passive film.
During the crossing of the chemical pickling unit the strip surfaces were kept moist in a room saturated with ambient-pressure steam.
At the outlet of this section, hence at the end of the finishing/passivating treatment, the strip appears completely pickled and having a good surface aspect. Reflection measuring conducted with a reflectometer (angle of reflection=60°) yielded values ranging from 30 to 35%. SEM investigations of the surface layer proved the absence of residual oxides thereon.
During strip processing, the automated systems for controlling bath concentrations duly kept the provided concentrations by adjusting fresh reagent flows and spent solution discharges.
There is exemplified the descaling, pickling and finishing treatment of a hot-rolled and annealed AISI 304L type austenitic steel coil, having 2.7 mm thickness, 1,270 mm width, 19.3 t weight.
Preliminarily to the treatment according to the present invention, the same strip was subjected to scale breaking by grit blasting. According to the present invention using the line described in example 1, operation was carried out under the following conditions: c=115 C/dm2, k=12.1 s and rate=55 m/min
At the outlet of this section the surface is free from oxide scale and residues of the dissolution products by effect of the joint action of the hydromechanical brushing and high-pressure rinsing treatment. For this product type the process was completed with the pickling treatment. After the electrolytic descaling treatment, the strip was subjected to the subsequent pickling and cleaning treatments in order to remove the chromium-depleted alloy layer and to generate a surface passive film.
At the outlet of this section, hence at the line outlet, the strip appears completely pickled and having a good surface aspect. SEM investigations of the surface layer proved the absence of residual oxides and of localized attacks. The roughness at the line outlet, detected transversally to the direction of rolling, exhibited average Ra values of about 2.4 μm.
During strip processing, the automated systems for controlling bath concentrations duly kept the provided concentrations by adjusting fresh reagent flows and spent solution discharges.
In this example there is described the descaling, pickling and finishing treatment of a hot-rolled and annealed AISI 316 austenitic steel coil having 3.0 mm thickness, 1,270 mm width, 19.3 t weight. Preliminarily to the treatment according to the present invention, the same strip was subjected to a scale breaking treatment by grit blasting. According to the present invention, using the line described in example 1, operation was carried out under the following conditions at a rate of 50 m/min.
At the outlet of this section the surface is free from oxide scale. However, brushing and high-pressure rinsing notwithstanding, some dissolution product deposits still appear on the strip surface. For this type of product only the pickling treatment was carried out. After the electrolytic descaling treatment the strip was subjected to the subsequent pickling and cleaning treatments in order to remove the chromium-depleted alloy layer and to generate a surface passive film.
At the end of the treatment, hence at the line outlet, the strip appears completely pickled and having a good surface aspect. SEM investigations of the surface layer proved the absence of residual oxides and of localized attacks. The roughness at the line outlet, detected transversally to the direction of rolling, exhibited average Ra values of about 2.6 μm.
During strip processing, the automated systems for controlling bath concentrations duly kept the provided concentrations by adjusting fresh reagent flows and spent solution discharges.
In this example there is illustrated the descaling, pickling and finishing treatment of a cold-rolled and annealed AISI 430 type ferritic steel coil, having 2.7 mm thickness, 1,270 mm width, 18.5 t weight. In this case the process constants are c=115 C/dm2 and k=10.0 s.
Preliminarily to the treatment according to the present invention, the same strip was subjected to a scale-breaking treatment by grit blasting. According to the present invention, using the line described in example 1, operation was carried out under the following conditions: c=115 C/dm2, k=10 s and rate=45 m/min.
At the end of the treatment the surface appeared free from oxide scale and dissolution product residues by effect of the joint action of the brushing and of the high-pressure rinsing. After the electrolytic descaling treatment the strip was subjected to the subsequent pickling, finishing/passivating and cleaning treatments in order to remove the chromium-depleted alloy layer and to generate a surface passive film.
At the end of the pickling and finishing treatment, hence at the line outlet, the strip appears completely pickled and having a good surface aspect. SEM investigations of the surface layer proved the absence of residual oxides and of localized attacks. The roughness at the line outlet, detected transversally to the direction of rolling, exhibited average Ra values of about 2.1 μm.
During strip processing, the automated systems for controlling bath concentrations duly kept the provided concentrations by adjusting fresh reagent flows and spent solution discharges.
In this example there is illustrated the descaling, the pickling and the finishing of a hot-rolled and annealed AISI 409 type ferritic steel coil, having 3.0 mm thickness, 1,020 mm width, 20.6 t weight.
Preliminarily to the treatment according to the present invention, the same strip was subjected to a scale-breaking treatment by grit blasting. According to the present invention, using the line described in example 1, operation was carried out under the following conditions: c=115 C/dm2, k=12,5 s and rate=40 m/min.
At the outlet of this section the surface appears free from oxide scale and dissolution product residues by effect of the joint action of the brushing and of the high-pressure rinsing. After the electrolytic descaling treatment the strip was subjected to the subsequent pickling, finishing/passivating and cleaning treatments in order to remove the chromium-depleted alloy layer and to generate a surface passive film.
At the outlet of this section, hence at the line outlet, the strip appears completely pickled and having a good surface aspect. SEM investigations of the surface layer proved the absence of residual oxides and of localized attacks. The roughness at the line outlet, detected transversally to the direction of rolling, exhibited average Ra values of about 1.7 μm.
During strip processing, the automated systems for controlling bath concentrations duly kept the provided concentrations by adjusting fresh reagent flows and spent solution discharges.
| Number | Date | Country | Kind |
|---|---|---|---|
| RM01A000747 | Dec 2001 | IT | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IT02/00810 | 12/29/2002 | WO | 7/8/2005 |
