SE462286B - SET FOR MANUFACTURE OF STAINLESS STEEL BANDS AND GOODS WITH GOOD SPACE AND GLOSS - Google Patents

Description

462 286 10 15 20 25 30 35 During exposure to the acidic beet solution, usually mixed acid solution of hydrofluoric acid (HF) and nitric acid (HNO3) at elevated temperature, a chemical attack also occurs on the oxide-free metal surface. When different chemical activities are present along the material surface, the metal dissolution takes place during an intermittent process, which means an enlargement of the microprofile of the strip surface, as can be seen from the following Fig. 1 and Fig. 2.

During the chemical pickling process, reaction products are formed which, due to the polar conditions present between these products and the metal surface, mean that said products are adsorbed to the enlarged microprofile of the strip. Studies and practical experience show that these reaction products as well as the accompanying residues from the beet bath have a high affinity for the band surface. This means that these cannot be completely removed during the rinsing processes.

When the strip in its pickled surface design is smooth-rolled, a sharp equalization of the elevations of the microprofile which takes place over the surface takes place and causes an overlap of said impurities in the surface layer. This means that the strip traditionally manufactured for embodiment 2B is given an insufficient surface condition due to said impurities in the surface layer. As a result, the steel can also not fully meet the requirements for surface unit and resistance to corrosion in several areas of use.

Another effect of the dirty surfaces is that the rollers in smooth rolling mills and alignment equipment are contaminated, which also impairs the surface purity and gloss of the strip surface.

DISCLOSURE OF THE INVENTION The present object has for its object to eliminate the said limitations and disadvantages in the production of belts for embodiment 2B. However, the method according to the invention is not limited to the production of strips for embodiment 2B but is also useful as a surface-improving insert in combination with conventional light rolling. According to the invention, these objects can be achieved by introducing certain processes for pickling and surface conditioning in combination with rolling direction as the final surface treatment. The invention has been developed in connection with attempts to improve surface smoothness and gloss of cold-rolled stainless steel strips after annealing and chemical pickling as well as electrochemical pickling in neutral electrolyte with subsequent chemical treatment. In these experiments, said processes were combined with an electrochemical surface conditioning in a weakly alkaline potassium sulphate solution, the strip surfaces showing the surprising effect that measuring values for surface smoothness and gloss were of the same order of magnitude and in some cases better than what is commonly stated for strips of finish 2B. underwent an additional manufacturing operation by smooth rolling.

Upon closer study of the treatment effect of the combined processes, it could be established that the prior art of electrochemical treatment in neutral aqueous solution of Na 2 SO 4 to remove oxides from annealing, working with six electrolysis cells with four anode positions and two cathode positions for the strip, yielded oxide-free surfaces for steel grades of Cr-Ni, Cr-Ni-Mo and Cr-Ni-Mo-Ti steels of average thickness 2.0 mm at the stated process data and processing time corresponding to belt speed 12 m / min.

The basic novelty of the method according to the invention is thus that an initial electrochemical and chemical pickling process for removing oxides from the previous annealing is combined with an electrochemical surface conditioning and rolling direction of the strip. Through the combined process technique, a metallically cleaner surface is obtained, which by controlled surface felling is given an improved surface smoothness and gloss, which gloss is further enhanced by a subsequent rolling direction.

A significant advantage of the method according to the invention is that the conditions for achieving requirements for surface smoothness and gloss according to embodiment 2B are given in direct connection with the pickling of the strip, after which subsequent rolling direction further improves the gloss of the strip surface. Of particular economic importance is that the method according to the invention does not require any additional manufacturing operation in the form of smooth rolling in a special rolling mill, as according to conventional methods. The invention thus relates to a combination process in the manufacture of stainless steel strips and plates with good surface smoothness and gloss starting from cold-rolled, annealed strips and plates, the novel features of the process being as follows: a) that the annealed strips or plates in a first step are subjected to an initial electrochemical pickling with alternating anodic and cathodic treatment in a neutral alkali metal salt solution, so that the oxides are removed from the annealing, b) that the bands or plates freed from oxide coating are then subjected to a second step. a surface felling of surface layers chromium-plated during the heat treatments of the strips or plates, which surface felling is carried out by treating the strips / plates with an acidic pickling solution which produces a chemical solution of the surface layers, which pickling solution consists of one or more inorganic or organic acids with possible addition of salts and / or surfactants, c) that they from oxides and defective surface layers in said first and said second steps, a) and b), the freed bands or plate surfaces are then subjected to an alternating anodic and cathodic electrochemical treatment in an aqueous solution of at least one alkali metal salt of pH> 7, which treatment involves a decapitation, ie a neutralization and rejection of impurities, pickling acid residues and acidic reaction products absorbed from the previous chemical treatment in said second step in the microprofile of the steel surface, and a slight removal of the protruding tips on the microprofile enlarged by the chemical surface felling of the standing surface. resulting in a surface-smoothing effect while simultaneously smoothing the steel surface, and d) the strips or plates after said electrochemical decapitation and surface conditioning in step c) undergo a rolling direction in roller straightening with polished rollers, resulting in an enhanced gloss of the steel surface. leads to a planing of the tapes or plates. The electrochemical treatment in step a) is suitably carried out with alternating anodic and cathodic treatment in a neutral sodium sulphate solution with a direct current of> 5 A / dm 2 at 60-90 ° C, preferably at 70-80 ° C. The treatment in step b) may consist of pickling in a mixed acid solution of hydrofluoric acid and nitric acid to remove incorrect surface layers, for example a mixed acid of 1.5 mol HF + 2.0 mol HNO 3 at the temperature 50-60 ° C and the treatment time 22-45 s , and the electrochemical decapitation and surface conditioning ic) is suitably carried out with direct current of current density> 10 A / dmz, the bands or plates occupying at least three anode positions and two cathode positions.

Preferably, the electrochemical decapitation and surface conditioning is carried out in an aqueous solution of potassium sulphate in a concentration of 50-200 g / l, preferably 75-150 g / l.

For the electrochemical surface conditioning in step c) very good results were obtained using an electrolyte with 100 g / l K 2 SO 4 of pH 10 and of the temperature 70 ° C with the addition of 0.5% surfactant and using seven electrolysis cells, the band assumed four anode positions and three cathode positions, after which the belt left the treatment in an anodic position to ensure freedom from hydrogen uptake and sufficient passivation effect. After 7.5 s anodic and also cathodic treatment at 20-25 A / dm? obtained after rinsing and drying clean surfaces with high gloss. The reduction of Cr6 + was effective and precipitated metal hydrates were easy to settle and filter.

According to a preferred embodiment of the invention, the strip surface is thus freed from oxides formed during annealing of the strip, by applying the known method of alternating anodic and cathodic treatment in an almost neutral, self-regenerating electrolyte of an aqueous solution containing 150-200 g / l Na2SO4, whereby the band obtains its polarity by contactless bipolar electric current transmission of a direct current with current densities of 10-15 A / dm? at a voltage of up to 30 V. Through the known anodic and cathodic processes, the oxides are removed from the strip surface partly by formation of sulfuric acid (H 2 SO 4) and press sulfuric acid (H 2 S 2 O 8) during evolution of oxygen at 462 286 10 15 20 25 30 in the anodic position and partly by the action of the alkali (NaOH) formed during reduction with hydrogen evolution when the belt is in the cathodic position, the metals in connection with the anodic reactions forming dissolved sulphates in the electrolyte which are spontaneously reacted with cathode formed NaOH to the electrolyte precipitated metal hydrates in solid form while simultaneously regenerating Na2SO4 in solution.

Through a continuous reduction of Cr6 + during the process, the treatment can be carried out under environmentally safe conditions with metal hydrates that can be filtered off from the electrolyte, which can be disposed of, as well as oxygen and hydrogen gas which can be released to free air during continuous dilution with large amounts of air.

After the treatment, the strip is freed from electrolyte and reaction products by air wiping and rinsing in a cascade during simultaneous brushing.

After the oxides have been removed, the strip surface can be subjected to a surface felling to remove chromium-plated surface layers with chromium contents lower than the requirements for steels obtained during heat treatments in the production of the strip, which surface felling according to an embodiment of the invention is by exposure to an acidic solution. the surface layer, which solution consists of one or more inorganic or organic acids with the addition of salts and / or surfactants, for example a stainless steel pickling bath consisting of 2 mol, hydrofluoric acid (HF) and 1.5 mol of nitric acid (HNO3) at temperature up to a maximum of 60 ° C.

A development of the method according to the invention is further characterized in that the strip surface freed from oxide and incorrect surface layer undergoes an electrochemical surface conditioning in an electrolyte consisting of potassium sulphate, K 2 SO 4 dissolved in water of concentrations up to saturated solution, preferably 100 g / l and of pH 8-10, which electrolyte can be added to a surfactant, eg a surfactant, by alternating anodic and cathodic action of a direct current corresponding to current densities ~ 10 15 20 25 30 35 462 286 up to 1 30 A / dmz at voltages up to 30 V, which direct current is applied contactlessly to the belt by bipolar electric current transmission and which treatment involves both a decapitation, ie a neutralization and rejection of the contaminants of acid acids and acidic reaction products adsorbed from previous chemical treatment in the microprofile of the belt surface and a light felling of the protruding tips of the micro-profile of the band surface enlarged by the chemical surface felling resulting in a surface-smoothing effect while simultaneously slightly smoothing the surface to gloss values comparable to embodiment 2B.

The processes of surface conditioning with the electrochemical pickling give similar basic reactions with anodic metal solution, formation of alkali at the cathode resulting in precipitation of metal hydrates which together with oxygen and hydrogen form the reaction products. The results that can be observed in the surface and its micro-profile indicate, however, that the phase boundary layer at the anode surface of the strip is affected by the composition and character of the electrolyte and there give different process conditions than in the electrochemical pickling.

The surface conditioning is carried out under environmentally safe conditions, partly through a continuous reduction of Cr6 +, as well as the disposal of the precipitated, solid metal hydrates. Soluble salts are washed out in connection with the filtration from the metal hydrates and returned to the process or can be disposed of for destruction.

After the surface conditioning, the excess electrolyte is preferably wiped off with compressed air, after which the clean strip surface can be rinsed in a cascade and dried with hot air.

Further features and aspects of the invention appear from the following claims and from the following description of examples.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 - illustrates an intermittent dissolution process during chemical pickling due to varying activity along the strip surface. 462 286 10 15 20 25 30 35 8 Fig. 2 shows a rough surface from chemical pickling.

Fig. 3 illustrates the treatment scheme according to the invention.

TESTS PERFORMED AND RESULTS The method according to the invention will be elucidated in connection with the following examples from experiments with samples from cold-rolled strips of widths 1800 mm and 2000 mm taken during operation after continuous recrystallization annealing. All test surfaces were covered with oxides.

All experiments were performed with the same treatments according to the process shown in the flow chart in Fig. 3.

PROCESS DATA ELECTROCHEMICAL Electrolyte 150 g / l Na2SO4 solution BEATING pH 6, 75 ° C current-tight 10 A / dm * Band polarity 4 anode positions 2 cathode positions Efficient 6 m (Anode 4.0 m process length (Cathode 2.0 m) Treatment time Adapted to a belt speed of 12 m / min = 5 s / cell CHEMICAL Beta solution 2.0 molar HN03 + BEETING 1.5 molar HF 60 ° C Treatment time 22 s = 1/2 betkar 4.5 m 45 s = 1 "9.0 m 90 s = 2 "18.0 m ELECTROCHEMICAL Electrolyte 100 g / l KZSO4 solution DECAPERATION AND pH ll, 70 ° C SURFACE CONDITION Current density 25 A / dn? Band polarity 4 anode positions 10 15 20 25 30 35 462 286 9 3 cathode positions Effective 3 m 1.5 m) process length (cathode 1.5 m) Treatment time The time was adapted to the belt speed 12 m / min Measurement of Surface uniformity Surface uniformity meter "Surtronic" Surface gloss Gloss meter "Reflectometer RB 60-M" ss 2333 (1s% cr-9% Ni) SS 2350 (17% Cr-11% Ni-2.2% Mo-Ti) Cargo thickness; 2.0 mm SS 2353 (17,5% Cr-12% Ni-2,7% Mo) Cargo thickness; 2.5 mm Treated steels RESULT - Electrochemical pickling led t ill oxide-free surfaces for all steel grades.

Subsequent chemical pickling, which concerned the removal of chromium-poor surface layers, gave a slightly lighter shade to the surfaces of samples treated for a short time, 22-45 s, while increased treatment time led to etching of the surfaces.

- Electrochemical surface conditioning led to clean shiny surfaces in all steels. SS 2350 also gave a shiny surface; however, slightly duller depending on the pattern that is structure-related to the titanium alloy steels.

Table 1 below shows surface smoothness and gloss values measured after surface conditioning before rolling direction. 462 286 lO 15 20 25 30 35 10 Table l Steel type Surface evenness Gloss value Ra pm Rf SS 2333 0.11 18 ss 2350 (Ti) 0.13 9-10 SS 2353 0.12 18-20 Common standard values for design 2B after smooth rolling are for 2.0-2.5 mm strip: Surface smoothness: Ra 0.2 μm max value Gloss: Rf 9 min value The results thus indicate that current standard values can be achieved according to the method of the present invention already in the surface treatment after annealing. The subsequent rolling direction according to the method of the invention provides a further improvement of the gloss of the surface by 5-6 units for the gloss value.

In summary, it can be said that reported results show that the method according to the invention gives a product which well corresponds to the high requirements of the embodiment 2B and eliminates the limitations which the conventional manufacturing methods are encumbered with.

The results also show that the method according to the invention makes it possible to produce the embodiment 2B for cold-rolled strips without smooth rolling as a final operation.

A consequence of the technique for the method according to the invention is that the surface treatment processes before the final step rolling direction are well suited for removing oxides from hot-rolled and annealed hot-rolled strips, whereby a pretreatment by sling cleaning and removal of residual products therefrom by high pressure rinsing with water can be applied before the electrochemical pickling. U 10 15 20 25 30 35 462 286 ll Utilization of the method according to the invention means that the hot-rolled strip obtains a smoother and cleaner surface which leads to improved quality after rolling which also favors the production of the embodiment 2B.

Claims (9)

462 286 12 lO 15 20 25 30 35 PATENTKRAV 1. In the production of stainless steel strips and sheets with good surface smoothness and gloss starting from cold-rolled, annealed strips and sheets, respectively, characterized by the following steps: a) that the annealed strips or sheets in a first step are exposed to a initial electrochemical pickling with alternating anodic and cathodic treatment in a neutral alkali metal salt solution, so that the oxides are removed from the annealing, b) that the strips or plates freed from oxide coating are then subjected in a second step to a surface removal of surface layers chromium heat treatments of the plates, which surface felling is performed by treating the strips / plates with an acidic pickling solution which produces a chemical solution of the surface layer, which pickling solution consists of one or more inorganic or organic acids with possible addition of salts and / or surfactants, c) those from oxides and defective surface layers in said first and said second steps, a) and b), free the strips or sheet metal surfaces are then subjected to an alternating anodic and cathodic electrochemical treatment in an aqueous solution of at least one alkali metal salt of pH> 7, which treatment involves partly a decapitation, i.e. a neutralization and rejection of from the previous chemical treatment in said second steps in the microprofile of the steel surface absorbed impurities, acid residues and acidic reaction products, and a slight felling of the protruding tips on the microprofile of the steel surface enlarged by the chemical surface felling, resulting in a surface-smoothing effect while simultaneously smoothing the steel surface or the plates after said electrochemical decapitation and surface conditioning in step c) undergo a rolling direction in rolling mills with polished rollers, resulting in a reinforced gloss of the steel surface, the rolling direction simultaneously leading to a planing of the strips or plates. 10 15 20 25 30 35 13 462 286 Process according to Claim 1, characterized in that the electrochemical treatment in step a) is carried out with alternating anodic and cathodic treatment in a neutral sodium sulphate solution with a direct current of> 5 A / dma at 60-90 ° C, preferably at 70-80 ° C. 3. A method according to claim 1, characterized in that the treatment in step b) consists of pickling in a mixed acid solution of hydrofluoric acid and nitric acid to remove defective surface layers. 4. A method according to claim 1, characterized in that the electrochemical decapitation and surface conditioning in step c) is carried out with direct current of current density> 10 A / dma, the bands or plates occupying at least three anode positions and two cathode positions. 5. A method according to claim 4, characterized in that the electrochemical decapitation and surface conditioning are carried out in an aqueous solution of potassium sulphate. 6. A method according to claim 4, characterized in that the electrochemical decapitation and surface conditioning are carried out in an aqueous solution containing alkali metal salt, preferably potassium sulphate, in a concentration of 50-200 g / l, preferably 75-150 g / l. 7. A method according to any one of claims 1-6, characterized in that the electrolyte for decapitation and surface conditioning is supplied with a surfactant for improved reactivity towards the strip surface. 8. A method according to any one of claims 1-7, characterized in that the transmission of the electric current to the belt during decapitation and surface conditioning takes place contactlessly by bipolar current transmission. 9. A method according to any one of claims 1-8, characterized in that each individual electrochemical and chemical treatment is followed by a rinsing of the belt, which is dried before rolling direction.