WO1990004047A1

WO1990004047A1 - Method relating to the manufacturing of cold rolled strips and sheets of stainless steel

Info

Publication number: WO1990004047A1
Application number: PCT/SE1989/000549
Authority: WO
Inventors: Bengt-Ivar Andersson; Peter Morsing
Original assignee: Avesta Aktiebolag
Priority date: 1988-10-13
Filing date: 1989-10-06
Publication date: 1990-04-19
Also published as: AU4329289A; SE8803636L; SE462286B; SE8803636D0

Abstract

The invention relates to a method of manufacturing strips and sheets of stainless steel. The strips are in a first step subjected to an electrochemical pickling, the oxides from the annealing being removed. Subsequently, those metal surface layers, which have been dechromed during the annealing, are removed through a pickling operation, wherein the surface layers are roughened. The strips, from which oxides and defective surface layers have been removed, subsequently in a third step are subjected to an alternatingly anodic and cathodic treatment, resulting in a neutralization and a slight removal of the protruding peaks of the microprofile of the steel surface, which has been magnified during the preceding step. Finally, the strips are roller straightened between bright rolls.

Description

Method relating to the manufacturinπ of cold rolled strips and sheets of stainless steel.

The present invention relates to a method of manufacturing col d rolled strips and sheets of stainless steel having a good surface smoothness and gloss, starting with colled rolled, annealed strips and sheets respectively.

BACKGROUND OF THE INVENTION

In the manufacturing of cold rolled strips and sheets of stainless steel, in the following jointly called strips, the manufacturing results in various surface finish conditions. There exist a number of standards for these surface conditions. One of these standardized surface conditions is called 2B in BS (British Standard) 1449 and ASTM A 480, which correspond to quality n or IIIC of DIN 17441. In the following reference 2B is used, B meaning bright.

2B bright finish is a technically and economically valuable standar¬ dized surface condition for strips of stainless steel and implies a bright surface, which is obtained through a light cold rolling pass of a prickled strip on polished rolls as an extra manufacturing step. The surfaces of 2.0-2.5 mm thick stainless strips, manufactured according to standard 2B, usually have a surface smothness of Ra max 0.2 μm and a surface gloss value of at least Rf 9.

The state of the art as far as the manufacturing of standard 2B is concerned has such technical limitations, that it cannot satisfy the high requirements which have to be raised as far as the surface condi¬ tion is concerned in order to fully utilize the technical properties of stainless steels , since the traditional way of manufacturing standard 2B is to remove oxides from the cold rolled strip after an annealing through a pickling process, in which the strip is treated with a pickling solution containing a mineral acid, is rinsed and dried and finally finish conditioned through skin passing.

During the treatment with the acid pickling solution according to the conventional technique, usually a mixed acid solution of hydrofluoric acid (HF) and nitric acid (HN0„) at an elevated temperature, a chemical attack on the oxide-free metal surface takes place. Since various chemical activities take place along the material surface, the metal dissolving takes place in an intermittent process, which means that the microprofile of the strip surface is magnified. During the chemical pickling process, reaction products are also produced, which, due to the polar conditions between these products and the metal surface, means that said products are adsorbed to the magnified microprofile of the strip. Tests and practical experiences show that these reaction products as well as accompanying residues from the pickling bath have a large affinity for the strip surface. This means that these materials cannot be removed during the rinsing processes. When the strip in its pickled surface condition subsequently is skin passed, i.e. smoothing-rolled, a strong flattening-out of the micro- profiled protrutions of the surface takes place, said microprotrutions being spread out over the surface and overlapping said impurities in the surface layer, which means that the strip, which is manufactured to standard 2B in the conventional way, obtains an insufficient surface condition due to said impurities in the surface layer. Consequently, the steel also cannot meet the requirements of surface purity and corrosion resistance in several ranges of application.

Another result of the impure surfaces is that the rolls of the skin passing mill and the straightening equipments are contaminated, which also impairs the surface purity and the gloss of the strip surface.

DISCLOSURE OF THE INVENTION

The object of the present invention is to eliminate the above- mentioned limitations and drawbacks, when strips of standard 2B are produced. However, the method according to the invention is not limited to the manufacturing of strips of standard 2B but can also be used as a surface improvement measure in combination with conventional smoothing-rolling. Said objects can be attained according to the invention through a combination of measurements when manufacturing cold rolled, annealed strips and sheets of a stainless steel, preferably to quality 2B, said strip through a continuous treatment being freed from oxides, which have been formed on the strip or sheet surface during the preceding annealing process, subsequent to which surface layers having an unsuitable chemical composition are removed through a subsequent, also continuous treatment, subsequent to which the strip or sheet surface is subjected to a pickling operation in order to remove the reaction products from a preceding treatment through a subsequent continuous, electrochemical treatment, which in the same process produces a smoothing effect with an improved surface smoothness and gloss, the strip and the sheet respectively also being subjected to a continuous roller straightening to a final finish.

An essential advantage of the method according to the invention is that the requirements as to surface smoothness and gloss according to standard 2B are met in direct connection with the pickling of the strip, subsequent to which the following roller straightening further improves the gloss of the strip surface. It is of particular economi- cal importance, that the method according to the invention does not require a particular skin passing mill, which is a very capital intensive production unit, and no extra manufacturing operation in the form of skin passing in a special skin passing mill, which is required according to conventional methods.

The initial pickling of the annealed strips or sheets is, according to the invention, performed as an electrochemical pickling with alterna- tingly anodic and cathodic treatment in a neutral alkali metal salt solution, so that the oxides from the annealing process are removed.

The strips and the sheets respectively, from which the oxide coating has been removed, are thereafter in a second step subjected to a removal of those surface metal layers which have been dechromed during the heat treatments of the strips and the sheets respectively, which surface metal removal is performed by treating the strips/sheets with an acid pickling solution, which chemically dissolves the surface metal layers, which pickling solution comprises one or several inorganic or organic acids with a possible admixture of salts and/or surface-active agents. The strip or sheet surfaces, from which oxides and defective metal surface layers have been removed in said first step and said second step, subsequently are subjected to alternatingly anodic or cathodic electrochemical cleaning-neutralization in a water solution of at least one alkali metal salt of pH>7, which treatment brings about a neutralization and removal of impurities, pickling acid residues and acid reaction products which from the preceding chemical treatment in said second step have been adsorbed in the microprofile of the steel surface, and also a slight removal of the protruding peaks of the microprofile of the steel surface which has been magnified at the chemical removal of the metal surface layer, resulting in a surface smoothing effect, and at the same time an improved brightness of the steel surface.

Subsequent to this electrochemical neutralization and surface condi¬ tioning treatment, the surface-conditioned strip passes a roller straightening mill having bright rolls, which results in a flattening of the strips or sheets and also in an additional improvement of the gloss of the strip or sheet surfaces.

Additional aspects of and characterizing features of as we 1 as advantages of the present invention are set forth in the attached patent claims and in the following description of preferred embodiment and obtained results.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will below be explained in more detail, reference being made to the accompanying drawings, in which:

Fig. 1 schematically shows a cross section of the surface layer on the starting material, greatly magnified;

Fig. 2 schematically shows the surface subsequent to an initial eletrochemical pickling; Fig. 3 schematically shows the appearance after a chemical dis¬ solving of the metal surface layer in the second process step;

Fig. 4 schematically shows the appearance of the surface layer after an eletrochemical neutralization and surface conditioning;

Fig. 5 schematically shows the appearance of the material after a roller straightening; and

Fig. 6 schematically shows the treatment schedule according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS In the following description it is assumed that the product to be treated are strips. However, it is apparent that the product also can be sheets.

In accordance with a preferred embodiment of the invention oxides are removed from the strip surface, which oxides were formed during the heat treatment of the strip, by employing the method, known per se, of alternating anodic and cathodic treatment in an almost neutral, self- generating electrolyte of an aqueous solution, which contains 150-200 g/1 Na_?S0 , the polarity of the strip being provided by a contactless bipolar electrical current conduction of a direct current having current densities of 10-15 A/dm² for a voltage of up to about 30 V, but the voltage may be higher.

Fig. 1 shows the starting material with its oxide layer 1 and its metallic surface layer 2, which during the heat treatment has been depleted of chrome, as well as its main material 3 of the steel strip, which has the required chemical composition. Through the anodic and cathodic process of the initial step a, Fig. 6, oxides 1 are removed from the strip surface on one hand through the formation of sulphuric acid (H_S0 ) and persulphuric acid (H₂S„0 ) , oxygen gas being formed at the strip surface, when the strip is in an anodic position, and on the other hand through the influence of the alkali (NaOH) and the reduction with a hydrogen gas evolution, which takes place, when the strip is in its cathodic position, the metals, in connection with the anodic reactions, forming sulphates, dissolved in the electrolyte,

_ which spontaneously react with NaOH, which is formed at the cathod, to

5 metal hydrates, precipitated in the electrolyte in a solid state, while at the same time being formed anew with Na SO. in solution.

Through a continuous reduction of Cr during the process the treat- _ ment can be performed, without endangering the environment, by means of metal, hydrates, which can be filtered from the electrolyte and disposed of, as well as oxygen gas and hydrogen gas, which while being continuously diluted with a large amount of air can be discharged to the ambient air. 5

Subsequent to the treatment the electrolyte and the reaction products are removed from the strip through air stripping and rinsing in cascade or in another way and suitably with a simultaneous brusching, step e. 0

The surface, from which the oxides have been removed, Fig. 2, is comparatively smooth but chemically defective. Thus, through the heat treatments, which the strip has been subjected to, i.a. in connection with the annealing, the metal surface layers 2 have been dechromed to a great extent, and therefor obtained chrome contents lower than those of the basic material 3 of the steel. The next step in the process, step b, Fig. 6 aims at removing these dechromed metal surface layers 2, which is done by treating the strips, from which the oxide coating has been removed, with an acid pickling solution, which results in a _Q chemical dissolving of metal surface layers 2, which pickling solution comprises one or several inorganic or organic acids with an admixture of salts and/or surface-active agents, e.g. a pickling bath for stain¬ less steel comprising a mixed acid of nitric acid (HNO- + hydrofluo¬ ric acid (HF). The molecular concentration should be adjusted to make 5 the HNO -concentration somewhat larger than the HF-concentration. In weaker baths the difference will decrease to a few tenths of the molecular concentration. The mixed acid may e.g. comprise 1.5 mol HF + 2 mol HNO at a temperature of not more than 60°C, suitably 50-60°C and the treatment time 22-45 seconds.

Another examplifying ambodiment of step b is that this treatment is done through an electrochemical influence, the surface-dechromed layer 2 being removed through an anodic dissolving in an acid or more neutral eletrolyte of e.g. an alkali metal salt, dissolved in water, or in a solution containing an inorganic or organic acid.

Subsequent to the treatment in step b the strip is rinsed in cascade or in another way, step f in Fig. 6, the concentrate being forwarded to a neutralization or a subsequent destruction.

When surface layers 2 are chemically dissolved in step b, the surfaces are somewhat "roughened". The appearance of such chemically "correct" but roughened metal surface layer 4 is shown schematically in Fig. 3. There is a typical irregular pattern in the microformat with alter¬ nating pronounced peaks and values. The third step in the process, step c, Fig. 6, aims at cutting the extreme peaks of surface profile 4 subsequently to the chemical pickling of step b as well as at removing the reaction products from the pickling in step b. This is done by subjecting the strip surfaces, from which oxides 1 and defective surface layers 2 in said first step a and second step b have been removed, to an alternating anodic and cathodic electrochemical treat¬ ment in an aqueous solution of at least one alkali metal salt, pH>7, suitably pH = 8-12, which treatment results in partly a picking, i.e. a neutralization and a knocking-off of impurities, pickling acid residues and axid reaction products, which have been adsorbed in the microprofile of the steel surface in said second step b, partly a slight cutting of the projecting peaks of microprofile 4 of the steel surface, which has been magnified due to the chemical surface cutting, resulting in a surface smoothing effect and at the same time a slightly more shining steel surface. As to the electrochemical neutra ization and surface conditioning, step c, very good results were obtained using an eletrolyte with 100 g/1 K SO , pH = 8-12, suitably about 10, and a temperature of 70°C, with an admixture of a surface-active agent and using seven electro¬ lytic cells, the strip being placed in four anodic positions and three cathodic positions, whereupon the strip left the treatment in an anodic position in order to prevent the strip from absorbing hydrogen and also to achieve a satisfactory passivation effect. After an anodic and also a cathodic treatment as 20-25 A/dm² for 7,5 seconds clean and brightly shining surfaces were obtained after rinsing and drying. The fifi--ii-- rreedduuccttiioonn ooff CCrr wwaass eeffffiicciieenntt aanndd tthhee pprecipitated metal hydrates could easily be settled and filtered off.

Furthermore, an embodiment of the method according to the present invention is characterized in that the strip surface, from which oxide 1 and defective metal surface layer 2 have been removed, in step c is subjected to an e ectrochemical neutralization and surface condition¬ ing in a electrolyte comprising potassium sulphate K SO , dissolved in water in concentrations as high as a saturated solution, preferably 00 g/1 and pH = 8-10, to which electrolyte a surface active agent can be added, e.g. a tenside, through alternatingly anodic and cathodic action of a direct current having current densities of up to ^L 30 A/dm² at voltages of up to about 30 V, which direct current is fed contactlessly to the strip through a bipolar e etrical current conduction.

The processes of the electrochemical neutralization and surface conditioning - step c - yield basic reactions similar to those of the electrochemical pickling, comprising an anodic metal dissolving, the formation of alkali at the cathode, resulting in a precipitation of metal hydrates, which jointly with oxygen and hydrogen gas constitute reaction products. However, the results, which can be observed on the surface and its microprofile, indicate, that the phase boundary layer at the anodic surface of the strip is influenced by the composition and properties of the electrolyte and in the area results in difference process conditions as compared to the electrochemical pickling.

The neutralization and surface conditioning is performed without endangering the environment on one hand through a continuous reduction of Cr and on the other hand through a disposal of the precipitated solid metal hydrates. Soluble salts are rinsed off in connection with the filtering from the metal hydrates and are recycled to the process or can be disposed of and be destructed.

Subsequent to the surface conditioning the electrolyte excess is stripped, suitably by means of pressurized air, and then the clean strip surface can be rinsed in cascade - step g - or in another way and be dried with hot air, step h, Fig. 6.

The appearance of the surface after the treatment in the third step c is shown in Fig. 4. Finally, in step d, the strips or sheets are, subsequent to the electrochemical neutralization and surface condi¬ tioning, subjected to a roller flattening in a roller straightening mill having bright rolls, step d, resulting in an improved gloss of the steel surface and flattened strips and sheets. The appearance of the smooth steel surface, obtained as a result of the integrated process has an improved surface gloss and is correct as to surface chemistry, and is shown schematically in Fig. 5.

PERFORMED TEST AND OBTAINED RESULTS

The method according to the invention will be described below with reference to the following examples, derived from tests with specimens from cold rolled strips having widths of 1800 mm and 2000 mm, taken from a production mill after a continuous recrystallization annealing.

All the specimens surfaces were covered with oxides, and in all the tests the same treatment steps were employed according to the process schedule, which is shown in the flow chart in Fig. 6. PROCESS DATA ELECTROCHEMICAL Electrolyte 150 g/1 Na SO E-solution, pH 6, 75°C

Current densi¬ 10 A/dm² ty

Strip polari¬ 4 anodic positions ty 2 cathodic positions

Efficient pro¬ 6 m (anode 4.0 ) cess length (cathode 2,0 m) Treatment The time was adjusted to a time strip speed of 12m/min =

5 s/cell

CHEMICAL PICKLING Pickling 2.0 mol. cone. HN0 + solution 1.5 " " HF

60°C

Treatment 22 s = 1/2 pickling time vessel 4.5 m 45 s = 1 - "" 9.0 m 90 s = 2 - "" 18.0 m

ELECTROCHEMICAL Electrolyte 100 g/1 K SO -solution NEUTRALIZATION AND pH 11, 70°C SURFACE CONDITIONING Current 25 A/cm² density

Strip 4 anodic positions polarity 3 cathodic positions

Efficient 3 m (anode 1.5 m) process length (cathode 1.5 m)

Treatment The time was adjusted to time the strip speed 12 m/min

Measurement of Surface Surface smoothness smoothness meter "Surtronic" Surface gloss Gloss meter "Reflektometer RB 60-M" Treated steel grades SS 2333 (18%Cr-9%Ni) SS 2350 (17%Cr-ll%Ni-

2.2%Mo-Ti) Material thickness: 2.0 mm SS 2353 (17.5 Cr-12%Ni-

2.7%Mo) Material thickness: 2.5 mm

RESULTS:

- The electrochemical pickling, step a, resulted in oxidefree surfaces for all the steel grades;

- The subsequent chemical dissolving treatment, step b, designed to remove chrome-depleted surface layers, resulted in a somewhat brighter shade for the surfaces of specimens treated for a shorter period of time, 22-45 seconds, while an increased treatment time resulted in an etching of the surfaces;

- The electrochemical neutralization and surface conditioning, step c, resulted in clean bright surfaces for all the steel grades. SS 2350 also developed a bright surface; however, it was somewhat duller due to its surface pattern, which has a structure typical for the titanium alloy steels;

- The roller straightening, step d, using bright rolles, provided an additional improvement of 5-6 gloss value units of the gloss of the surface.

The follwoing Table 1 shows surface smoothness and gloss value, measured after surface conditioning and roller straightening. TABLE 1 Steel grade Surface Gloss value smoothness After surface After roller Ra μm conditioning straightening Rf, about Rf, about

24

15

25

Normal values for standard 2B bright finish after smoothing rolling are for 2.0-2.5 mm strips:

Surface smoothness: Ra 0.2 μm maximum value

Gloss: Rf 9 minimum value

The results show, that through the method according to the present invention a product can be obtained, which very well meets the advanced requirements of standard 2B bright finish and eliminates the limitations, which the conventional methods of manufacturing suffer from.

Also, the results shown that through the method according to the invention it is possible to manufacture standard 2B bright finish for cold rolled strips without a smoothing rolling as a final step.

One result of the technique according to the method of the present invention is that the surface treatment processes prior to the final step of roller straightening are well designed to remove oxides from hot-rolled and annealed strips, and also it is possible to use a pretreatment including centrifugal cleaning and removal of residual products from the same by means of high pressure-flushing with water according to the invention prior to the electrochemical pickling.

Claims

PATENT CLAIMS

1. A method relating to the manufacturing of strips and sheets of stainless steel having a good surface smoothness and gloss, starting with cold rolled, annealed strips and sheets respectively, c h a r a c t e r i z e d by the following steps: a) that the annealed strips and sheets in a first step are subjected to an initial electrochemical pickling with alternatingly anodic and cathodic treatment in a neutral alkali metal salt solution in order to remove the oxides from the annealing process; b) that the strips and the sheets respectively, which have been freed from their oxide coating, in a second step are subjected to a removal of those metal surface layers which have been dechromed during the heat treatments of the strips and the sheets respectively, which metal surface removal is performed by treating the strips/sheets with an acid pickling solution, which brings about a chemical dissolving of the metal surface layers, which pickling solution comprises one or several inorganic or organic acids, possibly with an admixture of salts and/or surface-active agents; c) that subsequently the strips or the sheet surfaces, from which oxides and defective surface layers have been removed in said first and said second step, a) and b), are subjected to alternatingly anodic and cathodic eletrochemical treatment in an aqueous solution of at least one alkali metal salt of pH>7, which treatment involves a neutralization and removal of impurities, pickling acid residues and acid reaction products, adsorbed in the microprofile of the steel surface in the preceding chemical treatment in said second step, and also a slight removal of the protruding peaks of the microprofile of the steel surface, which has been magnified through the chemical metal surface removal operation, resulting in a surface smoothing effect and at the same time a slight improvement of the gloss of the steel surface; and d) that the strips or sheets subsequent to said electrochemical neutralization and surface conditioning in step c) are subjected to a roller straightening in a roller straightening mill having bright rolls, resulting in an improved steel surface gloss and flattening of the strips or the sheets.

2. A method according to claim 1, c h a r a c t e r i z e d in that the electrochemical treatment in step a) is performed with an alterna¬ tingly anodic and cathodic treatment in a neutral sodium sulphate solution with a direct current of > 5 A/dm² at 60-90°C, preferably at 70-80°C.

3. A method according to claim 1, c h a r a c t e r i z e d in that the treatment in step b) comprises a pickling in a mixed acid solution of hydrofluoric acid and nitric acid in order to remove defective surface layers.

4. A method according to claim 1, c h a r a c t e r i z e d in that the electrochemical neutralization and surface conditioning of step c) is performed in an aqueous solution of an alkali metal salt of pH 8-12.

5. A method according to claim 4, c h a r a c t e r i z e d in that the electrochemical neutralization and surface conditioning is performed in an aqueous solution of potassium sulphate.

6. A method according to claim 1, c h a r a c t e r i z e d in that the electrochemical neutralization and surface conditioning of step c) is performed with a direct current having a current density of > 10 A/dm² , said strips or sheets being held in at least three anodic positions and two cathodic positions.

7. A method according to claim 4, c h a r a c t e r i z e d in that the electrochemical neutralization and surface conditioning is performed in an aqueous solution containing an alkali metal salt, preferably potassium sulphate, in a concentration of 50-200 g/1, preferably 75-150 g/1.

8. A method according to any of claims 1-6, c h a r a c t e r i z e d in that a surface-active agent is added to the electrolyte, designed for said neutralization and surface conditioning, in order to improve the reactivity against the strip surface.

9. A method according to any of claims 1-7, c h a r a c t e r i z e d in that the feeding of the electrical current to the strip during the neutralization and surface conditioning is performed contactlessly through a bipolar current conduction for the used electrochemical processes.

10. A method according to any of claims 1-8, c h a r a c t e r i z e d in that each of the separate electrochemical and chemical treatments is followed by a rinsing of the strip, which is dried prior to the roller flattening.