BE1026911B1 - Method for producing stainless steel sheet finished in at least three different ways - Google Patents

Abstract

The present invention relates to a process for producing stainless steel sheet finished in at least three different ways including an HRAP (Hot Rolled Annealed and Pickled) finish, a 2D finish and a BA finish (high gloss or bright annealed - "Bright Annealed"). For all these finishes, the hot-rolled sheet steel is first annealed and pickled in a succession of pickling baths. For the production of sheet steel with the HRAP finish or the 2D finish, in some of the pickling baths, a mixed acid pickling agent containing hydrogen fluoride and nitric acid and / or sulfuric acid is used while the production of sheet steel with the BA finish requires to to use hydrochloric acid as a mordant in some of the pickling baths. To switch from one pickling bath configuration to another without having to shut down production and introduce recyclable replacement rolls into the production line, a special sequence is used when changing the pickling baths, making it possible to change continue to produce sheet steel with the 2D finish in the pickling bath configuration.

Description

The present invention relates to a method of producing stainless steel finished in at least three different ways, including an HRAP finish ("Hot Rolled Annealed and Pickled" ), a 2D finish and a BA finish (gloss or bright annealed - “Bright Annealed”). By means of a surface roller, the sheet steel with the 2D finish can be further finished into sheet steel with a 2B finish that is glossier than the matt sheet steel with the 2D finish The production of the finished sheet steel is based on rolls of hot-rolled, not yet pickled sheet steel, the so-called black rolls, which are unrolled and are each passed through at least a constant speed through at least one production line at least a first annealing furnace, a series of at least four consecutive pickling baths and at least one date rop subsequently contains a passivating pickling bath.

The process for producing the stainless steel sheet with the HRAP finish is carried out in the first production line. At least one of said rolls is unrolled hot-rolled, not yet pickled sheet steel and passed through the first production line at a substantially constant speed such that the sheet steel is successively annealed, pickled and passivated.

The annealing is intended to remove the stresses created in the steel during the hot rolling while the pickling is intended to remove the oxide skin that is on the hot rolled

D BE2019 / 5930 sheet metal. In addition, the low-chromium layer underneath must be stained away in such a way that a thin chromium oxide layer can be formed in the passivating pickling bath, which changes the stainless-steel character of the steel.

In the method of producing the stainless steel sheet with the 2D finish, the sheet steel produced in the first production line with the HRAP finish is further finished in a cold rolling mill and in a second production line. This second production line contains at least one further annealing furnace, a number of further successive pickling baths and at least one further passivating pickling bath. The sheet steel with the HRAP finish is thus cold-rolled and re-annealed, pickled and passivated.

Cold rolling of the sheet metal, with the thickness of the sheet steel decreased by at least 25%, produces a smoother surface compared to the HRAP finish. Re-annealing is necessary to relieve the stresses in the material and re-pickling is required to re-remove the oxide layer formed during annealing, after which the sheet metal must then be passivated again. The matt surface of the 2D finish thus obtained is then usually made more glossy by further treating the sheet steel with a so-called surface roller. In this way, a 2B finish with a smooth surface is obtained.

In the method for producing the stainless steel sheet with the BA finish, the sheet steel produced in the first production line with the HRAP finish is further finished in a cold rolling mill and in a third production line. This third production line contains at least one BA annealing furnace with a reducing atmosphere therein. The sheet steel with the HRAP finish is thus cold rolled and re-annealed. Due to the presence of the reducing atmosphere in the BA annealing furnace, only stresses are removed from the material without the material oxidizing in the BA annealing furnace. The steel is given a glossy surface, whereby autopassivation then automatically forms a thin oxide layer again, which does not, however, negate the glossy character of the material.

The following acids or combinations thereof are usually used for pickling stainless steel, in particular stainless chrome nickel steel: nitric acid (HNO3), hydrogen fluoride (HF), sulfuric acid (H2SO4) and hydrochloric acid (HCI). The action of these acids is described, for example, in the American Iron and Steel Institute's “Cleaning and descaling stainless steels” manual, which was reprinted in 1988 by the Nickel Development Institute.

As is apparent from the MER Notification of Aperam, of February 2012, these acids were also used by Aperam in Genk for the chemical pickling of the stainless chromium nickel steel. This publication indicates that pickling can be done with hydrochloric acid, but that this has the disadvantage that no in situ regeneration of hydrochloric acid is possible, so that the processing has to be done externally. The use of hydrochloric acid should thus be limited and it is in practice not possible to treat all stainless steel with hydrochloric acid. The mixed acid with hydrogen fluoride and nitric acid had the drawback of producing fluoride and nitrogen oxides. However, the emission of nitrogen oxides could be greatly reduced by mixing sulfuric acid and hydrogen peroxide in the pickling bath. The oxidizing nitric acid could be regenerated "in situ" by adding hydrogen peroxide. Free mordant could be regenerated by adding sulfuric acid to convert the weaker metal salts (fluorides, nitrates) to free acid (HF, HNO3) and metal sulfates. After all, the above-mentioned acids must be present in the pickling solution in their free or, in other words, in their acid form, in order to fulfill their pickling action.

In practice, depending on the type of finish, it is important for the quality of the sheet metal produced and for the production capacity to use a good combination of mordants or sequence of mordant paths. Because pickling and annealing lines are also very capital intensive, it is advantageous that one and the same pickling and annealing line can work with different pickling agents. It is of course always possible to lay the line still and to restart it after replacing the mordant in the different pickling baths.

Since this is a continuous production line, a roll of sheet steel must first be introduced into the line, which will be heated too strongly and pickled too strongly because it will stand still for a considerable time in the annealing furnace and pickling baths. Due to its adverse effects on the quality of the sheet steel, this roll can be reused several times, but will then have to be recycled. The drawback of the introduction of such a replacement roller is that it takes some time before it has been introduced into the production line and has been removed therefrom. This insertion and removal of the replacement roller thus increases the time that production stops. Moreover, the energy that is put into this role in the annealing furnace is completely lost. Because the pickling acids also act on this replacement roller, these are also used up further when the replacement roller passes through the pickling baths. A final disadvantage of using the replacement rollers is that they are not black rolls as these rolls have been annealed and pickled before. This changes the temperature development, which is created in the annealing furnace by means of gas burners. When a black roller is reinserted, which has considerably different absorption, reflection and emission values for heat compared to an already pickled roller, the temperature development in the annealing furnace will start to fluctuate and it will again take some time before a stable temperature development is obtained. is going to be.

Where in the past the production of sheet metal has been carried out in larger campaigns in order to avoid the disadvantages associated with the change of pickling baths, it has become increasingly important to be able to produce more to order, in order to avoid large stocks, whereby it is of course essential to overcome the disadvantages associated with the change of pickling baths.

The object of the invention is therefore to provide a new method for producing stainless steel sheet with different finishes which makes it possible to efficiently produce these different finishes and which further allows to change the pickling agents in the pickling baths without the production of the sheet metal must be shut down, and therefore without unnecessary waste of acids and energy.

For this purpose, the method according to the invention is characterized in that, for producing the stainless steel sheet with the HRAP finish, said pickling paths are in a first configuration, wherein said series of pickling baths is divided into a first series with at least the first three pickling paths and in a second series of at least one acid pickling bath and wherein the pickling baths of said first series of pickling baths each contain a mixed acid pickling agent based on at least hydrogen fluoride and sulfuric acid and / or nitric acid, wherein said pickling bath contains an acid pickling agent based on at least hydrogen fluoride and wherein the passivating pickling bath contains a passivating pickling agent based on at least hydrogen fluoride and nitric acid; that, for producing the stainless steel sheet with the BA finish, said pickling baths are in a second configuration, wherein the pickling baths of said first series each contain a hydrochloric acid pickling agent based on at least hydrochloric acid, wherein said pickling pickling pad is a further acid pickling agent based on contains at least hydrogen fluoride and wherein the passivating pickling bath contains a further passivating pickling agent based on at least hydrogen fluoride and nitric acid; that, to produce the stainless steel sheet with the 2D finish, said pickling paths are in said first or in said second configuration;

the method further comprising switching the production of stainless steel finished in one of said ways to the production of stainless steel finished in another of said ways and switching the first configuration of said pickling baths to the second configuration and vice versa,

that switching from said first configuration of the pickling baths to said second configuration comprises replacing said mixed acid pickling agent in said first series of pickling baths with said hydrochloric acid pickling agent, wherein replacing the mixing acid pickling agent in said first series of pickling baths with the hydrochloric acid pickling agent is performed sequentially upstream replacing the mixed acid pickling agent in each of said pickling baths with the hydrochloric acid pickling agent, starting with the pickling pad of said first series through which the sheet metal is passed last, and ending with the pickling bath through which the sheet metal is passed first; that switching from said second configuration of the pickling baths to said first configuration comprises replacing said hydrochloric acid pickling agent in said first series of pickling baths with said mixed acid pickling agent, wherein the replacement of the hydrochloric acid pickling agent in said first series of pickling pads by said mixing acid pickling agent is carried out in successive downstream replacement of the hydrochloric acid pickling agent in each of said pickling baths by the mixed acid pickling agent, starting with the pickling bath of said first series through which the sheet metal is passed first, and ending with the pickling bath through which the sheet metal is passed last; and that switching from the first configuration of said pickling paths in the first production line to the second configuration and vice versa is performed during pickling sheet metal in said pickling baths to produce said sheet metal with the 2D finish.

According to the invention, it was found that for the annealed BA finish, the best surface quality of the glossy sheet steel could be obtained by the second pickling bath configuration, namely by a succession of pickling baths in which the first series contains a hydrochloric acid based pickle . In particular, the hydrochloric acid mordant made it possible to better stain the grain boundary attack in order to obtain the desired gloss effect. For the 2D, or the 2B finish to be further produced therefrom, it was found that in order to obtain a good surface quality, it was possible to use the first configuration of pickling baths, namely to mix the acid with hydrogen fluoride and sulfuric acid and / or nitric acid. to be used in the first series of pickling baths and thus it was not necessary to use hydrochloric acid which is difficult to regenerate or recycle. For the HRAP finish, the use of the mixed acid of the first configuration in the successive pickling paths of the first series was found to be most suitable.

The use of a replacement roller to be recycled for switching from the first configuration of the pickling baths to the second and vice versa has in practice always resulted in an interruption of production by approximately eighty minutes. According to the invention, it was now found that, by making use of the specific monitoring of the pickling agents in the pickling baths, and by the specific sequence of their replacement, the production of the sheet metal should not be stopped during the replacement of the pickling agents in the pickling baths, but that production can continue during the configuration change, not with the production of sheet steel with the HRAP finish or the BA finish, but with the production of sheet steel with the 2D (or 2B) finish.

During the replacement of the pickling agent in a pickling bath of the first series, the pickling agent must first be removed from the pickling bath, after which the new pickling agent can be applied in the pickling bath. Preferably, this is sprayed onto the sheet steel so that it can act immediately without the pickling bath having to be filled first. The pickling is thus interrupted as briefly as possible. It has been determined that errors can occur on the surface of the sheet steel when the mordant, with the salts dissolved therein, dries on the surface of the sheet steel. These errors are avoided in the method according to the invention by always using the more strongly mordant hydrochloric acid pickling agent in the last pickling bath of the first series during the replacement of the mordant in the first series of pickling baths. This stronger pickling hydrochloric acid pickling agent was found to be able to largely eliminate the drying errors. When the mordant in the last bath of the first series is replaced, and the sheet metal in this last mordant bath can thus dry up, the subsequent acid mordant bath and the passivating mordant bath still remain to eliminate any drying errors. Preferably, this acid pickling batch and the passivating pickling bath contain sulfuric acid so as to provide a stronger pickling action.

Furthermore, these drying errors were found to be eliminated further during the subsequent cold rolling, annealing and re-pickling of the sheet metal in the second production line.

By producing sheet metal with the 2D (or 2B) finish during the configuration change, the sheet steel was always pickled because the pickling agent in the pickling paths contains the mixed acid and the stronger hydrochloric acid. Only when the mixed acid pickling agent is removed from the last pickling bath in the transition from the first to the second configuration is there one less pickling bath without there being a pickling bath with the stronger hydrochloric acid pickling agent and thus less pickling of the steel sheet. However, it has been found that this less strong pickling does not cause problems when the sheet metal is further cold rolled and subsequently annealed and pickled again. Preferably, the acid pickling agent in the acid pickling bath and the passivating pickling agent in the passivating pickling bath contain sulfuric acid during the changing of the pickling baths configuration so as to promote pickling.

In an embodiment of the method according to the invention, said passivating mordant is a mordant based on hydrogen fluoride, nitric acid and sulfuric acid and said further passivating mordant is a mordant based on hydrogen fluoride and nitric acid, whereby when switching said second configuration of the mordant paths to said first configuration said further passivating pickling agent is converted into said passivating pickling agent by adding at least sulfuric acid to said passivating pickling bath. Preferably, the replacement of the mixed pickling agent in the last bath of the first series with the hydrochloric acid pickling agent is performed in the presence of said passivating pickling agent in the passivating pickling bath.

Thus, in this embodiment, the pickling agent in the passivating pickling bath contains additional sulfuric acid when the pickling baths are in their first configuration in which pickling is mainly done with the less strong mixed acid pickling agent. Since said further passivating pickling agent can be easily transformed into said passivating pickling agent by adding sulfuric acid thereto, and because said passivating pickling agent can be easily transformed into said further passivating pickling agent by allowing the sulfuric acid to react during pickling, the passivating pickling bath should not be emptied and there can be no drying spots on the sheet metal in this last pickling bath.

Preferably, the replacement of the mixed pickling agent in the last bath of the first series with the hydrochloric acid pickling agent is performed in the presence of said passivating pickling agent in the passivating pickling bath.

The advantage of this is that the passivating pickling bath contains the stronger pickling passivating pickling agent with sulfuric acid when the last bath of the first series is emptied in such a way that any drying stains can hereby be better removed.

Preferably, during the production of the stainless steel sheet with the HRAP or 2D finish, with the pickling baths in said first configuration, sulfuric acid is metered into said passivating pickling bath to compensate for sulfuric acid reacting therein during pickling.

In this way, an improved pickling can be obtained without using the stronger pickling hydrochloric acid pickling agent in the first set of pickling baths.

In an embodiment of the method of the invention, said acid pickling agent is a hydrofluoric acid and sulfuric acid pickling agent and said further acid pickling agent is a hydrogen fluoride based pickling agent, wherein upon switching said second configuration of the pickling baths to said first configuration to said acid pickling bath sulfuric acid is added to convert said further acid pickling agent into said acid pickling agent.

In this embodiment, the pickling agent in the acid pickling bath contains additional sulfuric acid when the pickling baths are in their first configuration in which pickling is mainly done with the less strong mixed acid pickling agent. Since said further acid pickling agent can be easily transformed into said acid pickling agent by adding sulfuric acid thereto, and because said acid pickling agent can be easily converted into said further acid pickling agent by reacting the sulfuric acid during pickling, the acid pickling bath should not be emptied and in this penultimate pickling bath, there are thus no drying spots on the sheet metal.

Preferably, the replacement of the mixed pickling agent in the last bath of the first series with the hydrochloric acid pickling agent is carried out in the presence of said acid pickling agent in said acid pickling bath.

The advantage of this is that the acid pickling bath contains the stronger pickling acid pickling agent with sulfuric acid when the last bath of the first series is emptied in such a way that any drying stains can hereby be better removed.

Preferably, during the production of the stainless steel sheet with the HRAP or 2D finish, with the pickling baths in said first configuration, sulfuric acid is metered into said acid pickling bath to compensate for sulfuric acid which reacts away during pickling.

In this way, an improved pickling can be obtained without using the stronger pickling hydrochloric acid pickling agent in the first set of pickling baths.

In an embodiment of the method according to the invention, said mixed acid pickling agent, said acid pickling agent and said passivating pickling agent contain sulfuric acid, during the production of the stainless steel sheet with the HRAP or with the 2D finish, with the pickling baths in said first configuration, sulfuric acid in said pickling baths are metered into said acid pickling bath and said passivating pickling bath to compensate for sulfuric acid which reacts away during pickling.

In this way, the required pickling action of the mixed acids in the different baths during pickling can be maintained.

In an embodiment of the method according to the invention, the mixed acid pickling agent which is in the first configuration in the last or the last pickling baths of said first series is a mixed acid pickling agent based on hydrogen fluoride and sulfuric acid and the mixed acid pickling agent which is at least in the first configuration in the first or at least in the first two pickling baths of said first series there is a mixed acid pickling agent based on hydrogen fluoride, sulfuric acid and nitric acid.

In this embodiment, the pickling action of the first or the first two pickling baths is enhanced to optimally address the portion of the black oxide layer that is still on the sheet metal at that time.

In an embodiment of the method according to the invention, at least a part of the sheet metal with the 2D finish, and preferably almost all the sheet steel with the 2D finish, is passed through a surface roller at least once in order to obtain stainless steel with a 2B finish that has a shinier surface than the stainless steel sheet with the 2D finish. Preferably, the sheet metal has a thickness before the treatment with the surface roller which is not reduced, or at the most by 1%, by the treatment with said surface roller.

In this embodiment, the surface quality of the sheet metal is improved. A further annealing step is not necessary here because the internal structure of the sheet metal has not been changed by the surface roller.

In an embodiment of the method according to the invention, at least a part of the sheet metal with the BA finish, and preferably almost all the sheet steel with the BA finish, is passed through a surface roller at least once in order to obtain a shinier surface obtained. Preferably, the sheet metal has a thickness before the treatment with the surface roller which is not reduced, or at the most by 1%, by the treatment with said surface roller.

Also in this embodiment the surface quality of the already shiny sheet steel is further improved. A further annealing step is not necessary here because the internal structure of the sheet metal has not been changed by the surface roller.

In an embodiment of the method according to the invention, the sheet steel has a thickness before the cold rolling which is reduced by at least 25%, preferably by at least 30% and more preferably by at least 35% by cold rolling.

This cold rolling not only changes the internal structure of the sheet steel, but also smooths the surface of the sheet steel, resulting in a better surface quality compared to the HRAP finish. Both cold rolling and subsequent annealing and pickling of the sheet metal eliminate certain errors that may occur when switching from one pickling configuration to another.

In an embodiment of the method according to the invention, during the annealing and re-annealing of the steel sheet, the steel sheet is heated to a temperature of at least 800 ° C, preferably to a temperature of at least 900 ° C.

In an embodiment of the method according to the invention, said first series of pickling baths contains at least four pickling baths. The advantage of this embodiment is that when the pickling line is divided into more pickling baths, the length of the individual pickling paths is smaller, so that during the successive replacement of the pickling agent in these pickling baths, fewer problems with drying spots will occur. In an embodiment of the method according to the invention, the pickling baths of said first series have a length, the length of each of these pickling baths differing by no more than 20%, preferably not more than 10%, from the average length thereof. Preferably each of said pickling baths of the first series has a length of at most 25 m, preferably of at most 20 m.

Because no pickling bath of the first series has an exceptional length, this also ensures that the occurrence of drying stains is limited.

In an embodiment of the method according to the invention, the residence time of the sheet metal in each of the pickling baths of said first series during the switching from the first configuration of the pickling baths in the first production line to the second configuration or vice versa is at most 30 seconds and at preferably no more than 20 seconds.

In this embodiment it is also ensured that the occurrence of drying spots is limited. After all, the sheet metal in the different pickling baths only has a short opportunity to dry up when the pickling bath is emptied.

In an embodiment of the method according to the invention, production is carried out almost continuously with said first production line, whereby on average at least once a week, and in particular at least once every five days, the first configuration of the pickling baths in the first is switched over. production line on the second configuration or vice versa.

By regularly switching between the two configurations of the pickling baths, it is possible to produce all the different steel types and finishes to order, so that only a minimum amount of stocks must be maintained.

In an embodiment of the method according to the invention, said sheet steel is austenitic stainless sheet steel containing at least chromium and nickel.

The pickling agents used in the method according to the invention and in the above-mentioned embodiments thereof are particularly suitable for pickling austenitic stainless steel sheet.

In an embodiment of the method according to the invention, the mordant is sprayed onto the sheet steel in said pickling paths, the sheet metal preferably also being dipped in the mordant.

By spraying the mordant onto the sheet metal, the stained material is removed more efficiently, so that pickling can be done more efficiently, in particular when the sheet metal is also immersed in the mordant in combination therewith. Spraying the pickling agent on the steel sheet has the additional advantage that when replacing the pickling agent in a pickling bath, the steel sheet is not pickled in that pickling bath only for a short time that the pickling bath is emptied, for example for 15 minutes, and can dry therein, since the new pickling agent is immediately sprayed on the sheet metal and thus no minutes are lost for filling the pickling bath.

Further advantages and details of the invention will become apparent from the following description of some special embodiments of the method according to the invention.

This description is given with reference to the accompanying drawings in which:

Figure 1 schematically shows an overview of the first production line used in an embodiment of the method according to the invention for producing sheet steel with an HRAP finish;

Figures 2A to 2C schematically illustrate three different cold rolling used for cold rolling part of the sheet steel with the HRAP finish produced in the first production line shown in Figure 1;

Figure 3 schematically shows an overview of a first embodiment of a second production line used in an embodiment of the method according to the invention for further processing the cold-rolled sheet steel into sheet metal with a 2D finish;

Figure 4 schematically shows a surface roller for making the sheet steel smoother and more glossy, in particular for further finishing the sheet steel with the 2D finish obtained in the second production line according to Figure 3, to sheet steel with a 2B finish;

Figure 5 schematically shows an overview of a second embodiment of the second production line used in an embodiment of the method according to the invention for further processing of the cold-rolled steel sheet, which second production line corresponds to the second production line according to Figure 3 with inline a surface roller as shown in Figure 4, thus producing sheet metal with a 2B finish directly;

Figure 6 schematically shows an overview of a third production line, namely a blank annealing line, used in an embodiment of the method according to the invention for further processing the cold-rolled sheet steel into sheet steel with a BA finish; and Figure 7 shows two successive pickling baths from the first production line shown in Figure 1 with the associated reservoirs required to replace the pickling solution in both pickling baths.

The invention generally relates to a method of producing stainless steel sheet with different finishes. In particular, sheet steel with an HRAP finish (hot rolled, annealed and pickled - “Hot Rolled Annealed and Pickled”), sheet steel with a 2D finish (cold rolled, annealed and pickled) and sheet steel with a BA finish (cold rolled and bright annealed - "Bright Annealed") produced. Both the sheet metal with the BA finish and the sheet steel with the 2D finish are usually made more glossy by passing it through a surface roller. The matte 2D finish is thus converted into the more glossy 2B finish.

The stainless steel contains iron and at least 10.5% by weight of chromium. The carbon content is a maximum of 1.2% by weight. The stainless steel can further contain other elements such as nickel, molybdenum, titanium, manganese, copper, aluminum, etc. The stainless steel can be of the austenitic, ferritic or duplex type. The invention applies in particular to the production of stainless steel of the austenitic type, but can also be used in the production of the other steel types.

After the production of the stainless steel material itself in the various electric arc furnaces and the AOD refining units, this material is cooled and solidified in a continuous casting installation and burned to length. These slabs are, at a high temperature, hot rolled into rolls of sheet steel 1 with a thickness of, for example, 1.85 to 13.5 mm. Due to the high temperature to which the steel has been subjected, a relatively thick, black oxide layer has formed on the sheet metal. Furthermore, the steel has hardened during hot rolling and tensions have built up in it. The method according to the invention now relates to the further processing of these black rolls of sheet steel into the sheet steel with the finishes described above.

A first continuous production line that is used for this is shown schematically in figure 1. At the beginning of the line, the black rolls of sheet steel 1 are unrolled and welded together by means of a welding machine 2 to form a continuous strip of sheet steel 3. This continuous strip of sheet steel 3 is guided over different rollers 4 through the first production line. A number of the rollers 4 are arranged in an S-configuration in order to be able to exert sufficient grip and traction on the strip of sheet steel 3 and are each driven by means of electric motors 5 via a reduction gearbox 6. The speed of the electric motors 5 can be controlled to adjust the line speed, depending on the thickness and / or material of the sheet steel 3 passing through the production line. The gearbox 6 further allows shifting in large or small gear. The production line must always be shut down to change the gear.

In a first section of the production line, the sheet steel accumulator 7, the strip of sheet steel 3 is guided zigzag over rollers 4. The distance between these rollers 4 can be changed in order to accumulate more or less sheet steel in this first zone, such that, for instance during welding of the rolls of sheet steel 1, the sheet steel 3 still remains at a constant speed throughout the production line and in particular can continue to move through its annealing furnace.

After all, after the first section there is an annealing furnace 8 in the first production line in which the sheet steel 3 is heated to a temperature of at least 800 ° C, and more particularly to a temperature of at least 900 ° C. The temperature in the annealing furnace 8 is usually less than 1200 ° C. In the annealing furnace 8, the sheet metal 3 undergoes a certain temperature profile, as a result of which the stresses disappear from the sheet steel 3 and the desired internal structure is obtained.

After the annealing furnace 8, a guench section 9 follows in which the sheet steel 3 is cooled again by means of water and possibly air. The sheet steel 3 then passes through a dryer in which it is dried. After the dryer 10, an oxide breaker 11 follows in which the relatively thick, black oxide skin that is located on the sheet steel 10 is broken.

In a next step, the sheet metal 3 is passed through a blasting machine 12 with which metal grains are blasted on the oxide skin of the sheet steel 3 to make it loose and porous. The oxide skin is then mechanically removed as much as possible by means of brushes 13.

After an electrochemical pickling, in the electrochemical pickling paths 14, the sheet steel 3 is brushed again, with the brushes 15, and the sheet steel 3 is then rinsed, in the rinsing bath 16 in which the rinsing agent is sprayed on the sheet steel 3.

In the chemical pickling section, with six successive chemical pickling baths 17A-F, the sheet steel 3 is then pickled and passivated in the last pickling bath 17F. The last pickling bath is therefore a passivating pickling bath 17F. After the chemical pickling, the sheet steel 3 is again rinsed in a rinsing bath 55, dried in a dryer 18 and then zigzagged through an accumulator 19. In an inspection station 20, the quality of the steel can be monitored, after which the continuous strip of sheet steel 3 is cut into pieces again, with metal shears 21, and rolled into rolls 22 with the sheet steel with the HRAP finish. A part of these rolls 22 can be used as a final product, optionally after first cutting them into plates. The stainless steel sheet with the HRAP finish is matte and gray in color and has a fairly rough surface. The thickness of this sheet steel is the same as the thickness of the hot-rolled sheet steel, and is in particular included between 1.85 and 13.5 mm.

In the first place, this can be cold-rolled for the further finishing of the HRAP sheet steel. HRAP sheet steel with a thickness of up to 10 mm and preferably with a thickness of less than or equal to 8 mm is particularly suitable for cold rolling. By cold rolling, the thickness of the sheet steel 3 is firstly reduced to the desired final thickness. In particular, this thickness is reduced by 25 to 75%, and preferably by at least 30%, and more preferably by at least 35%. In addition to obtaining the desired final thickness, rolling has additional objectives such as obtaining good flatness, high surface quality and the desired mechanical properties (crystal structure). Different types of rollers can be used for cold rolling.

The first type is a quarto roll 23, which is shown schematically in Figure 2a. The roller group of such a roller consists of two working cylinders 24 between which the sheet metal 3 is rolled and two much larger supporting cylinders 25. The supporting cylinders 25 serve to absorb the enormous forces acting on the working cylinders 24. The desired decrease in thickness is obtained by passing the sheet steel 3 through this roller 23 one or more times, the sheet steel being unrolled and rolled in front and behind the sheet. To avoid damage to the sheet steel 3 and to the cylinders, the roll gap is cooled with an oil emulsion.

A second type of roller is a sexto roller 26 which is shown schematically in Figure 2b. This roller again contains two working cylinders 24 and two supporting cylinders 25, but an intermediate cylinder 27 is still arranged between the working and supporting cylinders. To avoid damage to the sheet steel 3 and to the cylinders, the roll gap is cooled with an oil emulsion.

The roller 28, shown in figure 2C, is a twenty roller roller and contains two working cylinders 24, eight support cylinders 25 and ten intermediate cylinders 27 between them. This roller 28 can be used by the optimum pressure distribution and by means of very finely ground working cylinders 24, in combination with clean roller oil instead of the oil emulsion, a high-quality surface quality can be obtained. This twintig roller 28 is used in particular for the production of sheet steel with the high-gloss or BA finish, but can also be used for the production of the sheet steel with the 2D or 2B finish.

After cold rolling, the sheet metal 3 must be re-annealed to remove the stresses from the material again and to obtain the desired crystal structure therein. This can happen in a second production line that is built approximately the same as the first.

The main parts of a possible embodiment of the second production line are shown in figure 3. These parts again include a welder 29, an accumulator 30, an annealing furnace 56, electrochemical pickling baths 31, chemical pickling baths 32A-F including a passivating pickling bath 32F, a dryer 33, an accumulator 34, and a metal shear 35. The sheet steel thus produced has a 2D finish, and has a rather matte appearance, and is rolled up into rolls 36.

To improve the surface quality of this sheet steel, the rollers 36 are further treated with a surface roller 37, shown in figure 4. This surface roller has only two working cylinders 38 which exert only a limited pressure on the sheet steel 3 such that the thickness of the sheet metal, or at least be reduced by less than 1%. Due to the action of the surface roller 37, the quality of the surface of the sheet metal can be reduced

3 are further improved and in particular the previously matte 2D finish is transformed into the more glossy 2B finish.

In order to make optimum use of the twintig roller 28, it is used not only in the production of the sheet steel 3 with the BA finish, but also in the production of part of the sheet steel with the 2D or 2B finish. In order to avoid stains on the sheet steel with the 2D or 2B finish as a result of the oil remaining on the surface thereof after cold rolling, a spraying installation 57 is provided for the further annealing furnace 56, with which an extra layer of oil is applied to the top surface of the cold-rolled sheet steel 3 is sprayed. The same oil or a different type of oil can be used here as it was applied to the working cylinders 24 of the cold roll 28 with which the relevant sheet steel 3 was cold-rolled. It was found that in this way the appearance of stains on the end product due to the presence of the rolling oil could be optimally avoided.

Even if the sheet metal is first cold-rolled in the quatro roller 23 or in the sexto-roller 26, in which an oil emulsion is always used, it is advantageous to apply a continuous layer of oil on the surface of the cold-rolled sheet steel before after all, it is also possible to prevent the occurrence of stains caused by the presence of the limited amount of oil-in-water emulsion on the surface of the sheet metal.

Figure 5 schematically shows a second embodiment of the second production line in which the surface roller 37 is incorporated at the back of the production line itself. In this way, sheet metal with the 2B finish is produced immediately and the rollers 36 therefore no longer have to be treated separately with the surface roller 37.

Figure 6 shows the main components of a third production line, namely a production line for producing sheet steel 3 with a high-gloss finish (BA finish). This third production line contains a welding machine 39, an accumulator 40, a BA annealing furnace 41 with a reducing atmosphere, in particular with an atmosphere based on hydrogen gas, a further accumulator 42 and a metal shears 43. The produced sheet metal with the BA finish is rolled up into rollers 44. These rollers 44 are preferably also treated with the surface roller 37 to improve the gloss and the surface quality of the sheet metal.

The rolls of sheet steel with the HRAP finish, the 2D finish, the 2B finish and the BA finish can be the end product as such, but can also first be cut into plates or strips. In the method of producing steel sheet with the different finishes, the first production line, shown in Figure 1, is used in the production of all the different finishes. The problem here is that the composition of the acids in the pickling baths 17A-F has to be adjusted. For the production of sheet steel with the HRAP finish, the 2D finish and the 2B finish, the pickling baths can use a mixed acid pickling agent, while for the production of sheet steel with the BA finish it is essential that the sheet steel in the first production line is treated with hydrochloric acid pickling agent. Such hydrochloric acid pickling agent can in theory also be used for the production of the other finishes, but in practice this is not possible due to the fact that it is not possible to process all the chlorinated waste water that would be created by this. In the method according to the invention, therefore, the composition of the pickling baths is changed regularly in order to be able to produce the different finishes alternately, on order. Figure 7 gives a schematic overview of the first two pickling paths 17A and B of the first production line. The other

24, BE2019 / 5930 pickling baths are constructed in the same way. As can be seen in more detail in this overview, the pickling baths are thus contained two by two in separate tanks 45. The pickling agent in both pickling baths is separated by a partition 46 such that it cannot flow from one pickling bath to another. The mordant itself is introduced into the mordant baths by spraying the mordant both at the top and at the bottom of the sheet metal by means of nozzles 47.

The mordant sprayed into the pickling baths, in particular in the first pickling bath 17A, according to the first configuration described below, is pumped via the discharge pipes 48 and the pump 49A into a settling tank 50A from which it overflows into a reservoir 51A. The mordant is then again sprayed from this reservoir 51A by means of a pump 52A through a conduit 53A into the mordant bath 17A. The sediment formed in the settling tank 50A can be pumped out by means of a pump 54A.

In order to be able to change the pickling agent in the pickling paths so as to obtain the second configuration described below, the same second system for the second pickling agent is provided, namely a discharge pump 49A ', a settling tank 50A', a reservoir 51A ', a pump 52A', a line 53A 'and a sedimentation pump 54A'. For the second pickling bath, also shown in Figure 7, and for the subsequent pickling baths, the same numbers are used for these elements, but with the letter corresponding to the letter of the pickling bath.

In the first configuration, namely in the configuration with the mixed acid pickling agent for the production of the HRAP, 2D or 2B finish, the first and second pickling baths 17A and B contain a mixed acid pickling agent containing mainly hydrogen fluoride, sulfuric acid and nitric acid, the third and fourth pickling pads 17C and D a mixed acid pickling agent containing mainly hydrogen fluoride and sulfuric acid,

the fifth pickling bath, namely said acid pickling bath 17E, an acid pickling agent again containing mainly hydrogen fluoride and sulfuric acid, and the sixth pickling bath, namely said passivating pickling bath 17F, a passivating pickling agent containing mainly hydrogen fluoride, nitric acid and sulfuric acid.

Through a connection (not shown) between the reservoirs 51A-E for this mixed acid pickling agent, the pickling agent is preferably flowed in cascade from the fifth 51E to the first reservoir 51A so as to be able to pickling in the fifth pickling bath 17E with purer pickling agent, i.e. pickling agent with less salts dissolved therein.

In each of the pickling seeds, the free acid content is preserved during the pickling by additionally adding sulfuric acid. In the first and second pickling bath, the nitric acid is dosed to enhance its pickling action.

In the second configuration, namely in the configuration with the hydrochloric acid pickling agent for the production of the BA finish, the first to the fourth pickling bath 17A-D contains a hydrochloric pickling agent with mainly hydrochloric acid, the fifth or said acid pickling bath 17E, a further acid pickling agent with mainly hydrogen fluoride, and the sixth or said passivating pickling bath 17F, a further passivating pickling agent containing mainly hydrogen fluoride and nitric acid.

To switch from the first to the second configuration or vice versa, the first production line is no longer shut down. The switch from the first to the second configuration or vice versa is carried out during the production of sheet metal with the 2D or 2B finish.

To switch from the first to the second configuration, one continues to produce sheet steel with the 2D or 2B finish. In a first step, the mixed acid pickling agent in the fourth pickling bath 17D is replaced by the hydrochloric acid pickling agent by first pumping the pickling bath into the reservoir 51D, which takes about fifteen minutes, and then the sheet metal in the fourth pickling bath 17D with the hydrochloric pickling agent from reservoir 51D '. to spray. Subsequently, the dosage of sulfuric acid in the acid pickling bath 17E and in the passivating pickling bath 17F can be stopped such that the sulfuric acid can be used therein to form the further acid pickling agent and the further passivating pickling agent with no or virtually no sulfuric acid. After all, the hydrochloric acid pickling agent in the fourth pickling bath 17D has a sufficiently strong effect at that time to sufficiently stain away any drying stains.

After all, in the next step, the third pickling bath 17C is emptied and the mixed acid pickling agent therein is replaced by the hydrochloric acid pickling agent. The same then happens for the second pickling bath 17B and finally for the first pickling bath 17A. As soon as the hydrochloric acid pickling agent is sprayed onto the sheet metal in the first pickling bath 17A, this sheet steel can be used for the production of sheet metal with the BA finish.

In order to switch from the second configuration to the first configuration again, production of sheet metal with the 2D or 2B finish is started again. In a first step, the hydrochloric acid pickling agent in the first pickling pad 17A is replaced by the mixed acid pickling agent by first pumping the pickling bath into the reservoir 51A ', which takes about fifteen minutes, and then by removing the sheet metal in the first pickling additive 17A with the mixed acid pickling agent from the reservoir. 51A to be sprayed. In the next step, the second pickling bath 17B is emptied and the hydrochloric acid pickling agent therein is replaced by the mixed acid pickling agent. The same then happens for the third pickling bath 17C and finally for the fourth pickling bath 17D.

Before the fourth pickling bath 17D is emptied, sulfuric acid is added to the acid pickling bath 17E and the passivating pickling bath 17F, preferably when the hydrochloric acid pickling agent in the third pickling bath 17C is replaced by the mixed acid pickling agent. In this way, the acid pickling bath 17E and the passivating pickling bath 17F have a stronger pickling action when the fourth pickling bath 17D becomes empty and thus dry spots can be formed on the sheet metal. After all, these drying spots are already at least partially eliminated by the stronger pickling action of the latter two pickling baths 17E and F.

As soon as the mixed acid pickling agent is sprayed onto the sheet steel in the fourth pickling bath 17D, this sheet steel can no longer be used for the production of sheet steel with the 2D or 2B finish, but also for the HRAP finish.

Inspection of the produced sheet steel with the different finishes showed that during the exchange of the pickling bath configuration in the manner described above, no significant quality deviations of the sheet steel produced during the exchange with the 2D or the 2B finish could be detected, and that the prescribed quality requirements have been met. However, if sheet metal with the BA finish, or with the HRAP finish, was produced during the exchange, quality deviations were found, which in particular for the BA finish were such that the prescribed quality requirements were no longer met. The same was noted when the replacement of the pickling solutions in the different pickling baths was not carried out in the order described above.

For the tests carried out, the different pickling baths each had a length of approximately seventeen meters. Because the sheet steel with the 2D or 2B finish had a thickness of less than 8 mm, and was thus guided relatively quickly through the first production line, its residence time in the different baths varied between 15 and 30 seconds.

Claims (21)

CONCLUSIONS 1. Method for producing stainless steel sheet finished in at least three different ways, including an HRAP finish (“Hot Rolled Annealed and Pickled”), a 2D finish and a BA finish (“Bright Annealed”), in which method rolls (1) are hot-rolled, not yet pickled sheet steel, which are unrolled and are each guided at substantially constant speed through at least one first production line, said first production line comprising at least one annealing furnace (8), a series of at least four consecutive contains pickling baths (17A - E) and at least one subsequent passivating pickling bath (17F), the process for producing the stainless steel sheet with the HRAP finish in said first production line and comprising the following steps: - providing at least one of said rolls (1) hot-rolled, not yet pickled sheet steel (3); - unrolling the roll (1) of sheet steel; - guiding the unrolled sheet steel (3) at a substantially constant speed through said first production line for successively annealing, pickling and passivating the unrolled sheet steel, and - removing the thus obtained sheet steel from the first production line with the HRAP finishing, the process for producing the stainless steel with the 2D finish in said first production line and further in a cold rolling mill (23, 26, 28) and in a second production line containing at least one further annealing furnace (56) , contains a number of further successive pickling paths (32A - E)) and at least one further passivating pickling bath (32F), and comprises the following steps: - providing at least one of said rolls (1) hot-rolled, not yet pickled sheet steel ( 3); - unrolling the sheet metal roll; - guiding the unrolled sheet steel (3) at a substantially constant speed through said first production line for successively annealing, pickling and passivating the unrolled sheet steel; - cold rolling of the passivated sheet steel; re-annealing, pickling and passivating the cold-rolled steel sheet in said second production line; and - removing the thus obtained sheet steel with the 2D finish from the second production line, the process for producing the stainless steel sheet with the BA finish being carried out in said first production line and further in a cold rolling mill (28) and in a third production line, without pickling baths but with at least one BA annealing furnace (41) containing a reducing atmosphere, and comprising the following steps: - providing at least one of said rolls (1) hot-rolled, not yet pickled sheet steel (3); - unrolling the sheet metal roll; - guiding the unrolled sheet steel (3) at a substantially constant speed through said first production line for successively annealing, pickling and passivating the unrolled sheet steel; - cold rolling of the passivated sheet steel; re-annealing the cold-rolled steel sheet under said reducing atmosphere in said third production line; and - removing from the third production line the thus obtained sheet metal with the BA finish, characterized in that, for producing the stainless steel sheet with the HRAP finish, said pickling paths (17A-E) are in a first configuration, wherein said series of pickling baths is divided into a first series having at least the first three pickling baths (17A-E) D) and in a second series of at least one acid pickling bath (17E) and wherein the pickling baths (17A-D) of said first series of pickling baths each contain a mixed acid pickling agent based on at least hydrogen fluoride and sulfuric and / or nitric acid, wherein said acid pickling bath (17E ) contains an acid pickling agent based on at least hydrogen fluoride and wherein the passivating pickling bath contains a passivating pickling agent based on at least hydrogen fluoride and nitric acid; that, for producing the stainless steel sheet with the BA finish, said pickling baths (17A-E) are in a second configuration, wherein the pickling baths (17A-D) of said first series each include a hydrochloric acid pickling agent based on at least hydrochloric acid wherein said acid pickling bath (17E) contains a further acid pickling agent based on at least hydrogen fluoride and wherein the passivating pickling bath contains a further passivating pickling agent based on at least hydrogen fluoride and nitric acid; that, to produce the stainless steel sheet with the 2D finish, said pickling baths (17A-E) are in said first or said second configuration; that the method further comprises switching the production of stainless steel finished in one of said ways to the production of stainless steel finished in another of said ways and switching the first configuration of said pickling baths (17A-E) to the second configuration and conversely, switching from said first configuration of the pickling baths (17A-E) to said second configuration comprises replacing said mixed acid pickling agent in said first series of pickling baths (17A-D) with said hydrochloric acid pickling agent, replacing the mixed acid pickling agent in said first series of pickling baths (17A-D) carried out by the hydrochloric acid pickling agent is successively upstream replacing the mixed acid pickling agent in each of said pickling baths (17A-D) with the hydrochloric acid pickling agent, starting with the pickling bath (17D) of said first sheet where the sheet metal (3) is passed through last, and finish d with the pickling bath (17A) through which the sheet metal is led first; switching from said second configuration of the pickling baths (17A-E) to said first configuration comprises replacing said hydrochloric acid pickling agent in said first series of pickling baths (17A-D) with said mixed acid pickling agent, replacing the hydrochloric acid pickling agent in said first series of pickling baths (17A-D) is carried out by the mixed acid pickling agent by successively downstream replacing the hydrochloric acid pickling agent in each of said pickling baths (17A-D) with the mixing acid pickling agent, starting with the pickling bath (17A) of said first series where the sheet metal ( 3) is passed first, and ends with the pickling bath (17D) through which the sheet metal is passed last; and that switching from the first configuration of said pickling baths (17A-E) in the first production line to the second configuration and vice versa is performed during pickling of sheet steel (3) in said pickling baths (17A-E) for the production of said sheet steel with the 2D finish. Method according to claim 1, characterized in that said passivating mordant is a hydrofluoric acid, nitric acid and sulfuric acid mordant, and said further passivating mordant is a hydrofluoric acid and nitric acid mordant, wherein when switching from said second configuration the pickling baths (17A-E) in said first configuration, said further passivating pickling agent is transformed into said passivating pickling agent by adding at least sulfuric acid to said passivating pickling bath. Method according to claim 2, characterized in that the replacement of the mixing pickling agent in the last bath (17D) of the first series by the hydrochloric acid pickling agent is carried out in the presence of said passivating pickling agent in the passivating pickling bath (17F). Method according to claim 2 or 3, characterized in that during the production of the stainless steel sheet with the HRAP or with the 2D finish, with the pickling baths (17A-E) in said first configuration, sulfuric acid in said passivating pickling bath (17F ) is dosed to compensate for sulfuric acid which reacts away during pickling. Process according to any one of claims 1 to 4, characterized in that said acid pickling agent is a hydrofluoric acid and sulfuric acid pickling agent and said further acid pickling agent is a hydrogen fluoride based pickling agent, wherein when switching said second configuration of the pickling baths (17A-E) is added to said acid pickling bath (17E) sulfuric acid at said first configuration to convert said further pickling agent to said pickling agent. A method according to claim 5, characterized in that the replacement of the mixed pickling agent in the last bath (17D) of the first series is carried out by the hydrochloric pickling agent in the presence of said acid pickling agent in said acid pickling bath (17E). Method according to claim 5 or 6, characterized in that during the production of the stainless steel sheet with the HRAP or the 2D finish, with the pickling baths (17A-E) in said first configuration, sulfuric acid in said pickling pad (17E) is dosed to compensate for sulfuric acid which reacts away during pickling. Method according to any one of claims 1 to 7, characterized in that said mixed acid pickling agent, said acid pickling agent and said passivating pickling agent contain sulfuric acid, wherein during the production of the stainless steel sheet with the HRAP or 2D finish, with the pickling baths ( 17A-E) in said first configuration, sulfuric acid is metered into said pickling paths (17A-E) and into said passivating pickling bath (17F) to compensate for sulfuric acid reactive during pickling. A method according to any one of claims 1 to 8, characterized in that the mixed acid pickling agent contained in the first configuration in the last or last pickling seams (17C and D) of said first series is a hydrofluoric acid and sulfuric acid mixed pickling agent and that the mixed acid pickling agent located in the first configuration at least in the first or at least in the first two pickling baths (17A and B) of said first series is a mixed acid pickling agent based on hydrogen fluoride, sulfuric acid and nitric acid. Method according to any one of claims 1 to 9, characterized in that at least a part of the sheet metal with the 2D finish, and preferably almost all the sheet steel with the 2D finish, at least once through a surface roller (37 ) in order to produce stainless steel sheet with a 2B finish that has a shinier surface than the stainless steel sheet with the 2D finish. Method according to any one of claims 1 to 10, characterized in that at least a part of the sheet metal with the BA finish, and preferably substantially all the sheet steel with the BA finish, at least once through a surface roller (37 ) is obtained in order to obtain a shinier surface. Method according to claim 10 or 11, characterized in that before the treatment with the surface roller (37), the sheet steel (3) has a thickness which is not reduced or at the most by 1% by treatment with said surface roller. Method according to any one of claims 1 to 12, characterized in that before the cold rolling the sheet steel (3) has a thickness which by cold rolling by at least 25%, preferably by at least 30% and more preferably by at least Is reduced by 35%. Method according to any one of claims 1 to 13, characterized in that during the annealing and re-annealing of the sheet metal, the sheet metal is heated to a temperature of at least 800 ° C, preferably to a temperature of at least 900 ° C C. Method according to any one of claims 1 to 14, characterized in that said first series of pickling baths (17A-D) contains at least four pickling paths (17A-D). Method according to any one of claims 1 to 15, characterized in that the pickling baths (17A-D) of said first series have a length, the length of each of these pickling baths (17A-D) not exceeding 20%, at preferably not more than 10%, deviates from its average length. Method according to any one of claims 1 to 16, characterized in that each of said pickling baths (17A-D) of said first series has a length of at most 25m, preferably at most 20m. Method according to any one of claims 1 to 17, characterized in that the residence time of the sheet metal in each of the pickling paths (17A-D) of said first series during the switching of the first configuration of the pickling baths in the first production line to the second configuration or vice versa is at most 30 seconds and preferably at most 20 seconds. Method according to any one of claims 1 to 18, characterized in that production is virtually continuous with said first production line, with an average changeover from the first at least once a week, and in particular at least once every five days. configuration of the pickling paths (17A-E) in the first production line on the second configuration or vice versa. Method according to any one of claims 1 to 19, characterized in that said sheet steel is austenitic stainless sheet steel containing at least chromium and nickel. Method according to claim 20, characterized in that the pickling agent is sprayed onto said sheet steel (3) in said pickling baths (17A-E), the sheet steel preferably also being immersed in the pickling agent.