FR2781704A1 - PROCESS AND PLANT FOR MANUFACTURING A HOT ROLLED TAPE COATED BY ELECTRODEPOSITION - Google Patents

Abstract

The present invention relates to a method and an installation for manufacturing a hot rolled strip coated by electrodeposition. The process is characterized in that the hot rolled strip is passed continuously, in line, through a unit. chemical pickling (2) and then in an electroplating unit (10) comprising electrolysis cells.

Description

The present invention relates to a method and an installation for

manufacturing

  a hot rolled strip coated by electrodeposition.

  When manufacturing steel products, a coating is often reported on the finished product. This coating essentially serves to protect the product from corrosion. To a large extent, coatings of zinc or zinc alloys have been used for this purpose, but also coatings of tin or chromium which

  have been of great interest in the field of packaging tapes.

  For the continuous coating of steel strips, two of the most common methods have been used, namely fire tinning and electroplating (electroplating). In both cases, a cold rolled strip is used as the raw material. This cold rolled strip is obtained in cold rolling mills from a thicker raw material (hot rolled strip), the typical thickness of

  the strip going from 1.5 to 5 millimeters to a final thickness of 0.3 to 2 millimeters.

  Before the hot rolled strip can be rolled, the scale (a surface layer made up of different iron oxides and having a typical thickness of 5 to 20 microns) which is formed during the hot rolling process, must be removed. This elimination is generally carried out by dissolving this layer in mineral acids such as sulfuric acid or hydrochloric acid. This process is called pickling and is carried out in pickling installations with continuous or discontinuous operation. An example of implementation of such a method

  is described for example in patent AT N 399 517.

  Attempts have been made to combine individual steps in the manufacture of steel strips in order to save on the cost of the installations required and to improve the quality of the product obtained. For example, it is known in the art to integrate the aforementioned pickling and cold rolling processes. Patent DE N l 960 6305 also describes a possibility of implementing the hot rolling process and the

  pickling process in the same installation.

  By the cold rolling process described, hardening of the material is obtained which must be removed again by a subsequent annealing process. This annealing process is carried out discontinuously in a bell furnace or continuously in a passing furnace. In the case of fire galvanizing, the annealing process and the deposition process are implemented jointly in a strip processing installation in which the annealing process is first carried out and then, during another step, the strip is covered with a layer of zinc by immersion in

molten zinc.

  Another possibility of depositing zinc and other metals on a steel strip is to use the electrolytic process. In this case the strip, most of the time an annealed cold-rolled strip is drawn through a bath which contains metal ions in solution. The strip is connected, by suitable methods, to the negative pole, that is to say to the cathode of a rectifier, while facing the strip are arranged metal anodes which are connected to the positive pole of the rectifier. Following the passage of an electric current between these poles, the metal ions are deposited on the strip in

steel in metallic form.

  Such a process is described for example in Austrian patent AT 373,922. Another process which is particularly suitable for the deposition of zinc alloys from a chloride-based electrolyte, while the space of the anodes is separated from space

  cathodes by a diaphragm, is described in patent EP 580 730.

  In recent years the manufacturing process for a hot rolled strip has undergone significant development and it is now possible to directly produce, by hot rolling, an increasingly thin strip. As a result, it becomes possible to use this hot rolled strip in many applications which were previously reserved for a cold rolled strip. However, this results in

  need to provide corrosion protection already on the hot rolled strip.

  In Japanese patent JP 91 258210, it has been proposed for this purpose to descaling, in the dry state, a hot rolled strip and then to execute a hot zincing process. However, this process has some drawbacks because dry descaling processes are necessary and the combination, which is advantageous for a cold rolled strip, of annealing and fire galvanizing processes no longer makes sense since it is not

  no need to anneal the hot rolled strip.

  The process according to the invention is characterized in that the hot-rolled strip is passed in a continuous manner, online, through a chemical pickling unit and then through an electroplating unit comprising electrolysis cells. Another advantageous embodiment of the invention is characterized in that

  rinse the strip between the pickling and electroplating units.

  Another advantageous embodiment of the invention is characterized in that hydrochloric acid is used as pickling liquid and, as electroplating liquid, a chloride-based electrolyte and which can also be used, as pickling liquid, sulfuric acid and, as liquid

  electroplating, a sulfate-based electrolyte.

  Another advantageous embodiment of the invention is characterized in that the solution formed during pickling, containing iron, is used as

liquid for plating.

  Another advantageous embodiment of the invention is characterized in that, as a pickling solution, is used in at least one stage of the

  pickling, the solution leaving the electrolysis cells.

  The invention also relates to an installation for manufacturing a hot-rolled strip coated by electrodeposition, characterized in that a chemical pickling unit is connected in line with an electrodeposition unit comprising electrolysis cells and the chemical pickling unit and the electrolysis cells can

  be separated by a ringing unit.

  The advantage of this process is that the two processes are carried out at relatively low temperatures of 50 to 90 C, in aqueous solutions. Therefore it is no longer necessary to heat the entire mass of the steel strip to

  temperatures ranging from 300 to 500 C, which results in significant energy savings.

  In the previous method of manufacturing a steel strip coated by electrodeposition, it is also always necessary to carry out, as a pretreatment of the strip, a short pickling in acid, mainly to ensure good adhesion of the layer. deposited on the base material. In addition, it is necessary to provide, in the prior art, for degreasing because the steel strips are oiled in order to protect them from corrosion during their intermediate storage. These process steps can also be omitted because, in the process according to the present invention, pickling and electroplating are carried out directly one after the other, possibly being separated by a rinsing operation. We obtain

  thus saving chemicals and reducing waste water production.

  The solution of iron sulphate or of iron chloride produced by pickling can also be used in part, in all cases, after the execution of a purification process, as described for example in the Austrian patent AT 400 042, to serve as a source of iron in an electrolyte used for the deposition of an alloy coating, such as for example a zinc-iron alloy. The process for producing such a coating in a chloride-based electrolyte is described for example in the Austrian patent AT

397,663.

  An embodiment of the present invention will be described below, by way of non-limiting example, with reference to the appended drawing which is a diagram of an installation for manufacturing a hot rolled strip coated by electrodeposition according to the present invention. An untreated hot rolled strip 1 is brought to a chemical pickling unit 2 which comprises for example two pickling baths 3,4. This pickling unit 2 is supplied with fresh acid 5 at the point of exit from the strip. The pickling unit is shown as using a single pickling acid, but different acids could also be used in the pickling baths 3,4. The spent acid 6 is discharged at the point of entry of the strip and it is transmitted to an acid purification unit 7. The pickling acid 8 which leaves therefrom can be transmitted to a regeneration of the acid so that it can be reintroduced later in the acid baths, as fresh acid. After passing through the chemical pickling unit 2, the strip is freed, in a rinsing unit 9, of the acid adhering thereto and it is then transmitted directly to electrolysis cells 10 in order to be coated there. by electrodeposition. The finished coated strip 11 then leaves the installation and can be

then wound into a reel.

  The electrolyte 12 is supplied from a dissolving unit 13 in which the electrolyte is prepared. For this purpose, use is made of the solution 14 of iron sulphate or of iron chloride produced by the pickling, after its passage through the unit 7

  acid purification, and a complementary zinc supply 15.

  We will now give some examples of implementation of the invention. In a pilot installation intended for the electrolytic coating of metal strips, which consists essentially of unwinding, degreasing, pickling, electrolysis cells, rinsing, drying, and winding devices, has installed coils of a hot rolled strip, thin and

  coated with scale, 300 mm wide and 1 mm thick.

  The degreasing unit has not been put into service.

  NI test: The hot-rolled strip was treated in the pickling unit with a sulfuric acid solution containing 20% H12SO4 at a temperature of 90 C. At the end of the pickling phase , the strip was rinsed with water for a short period of time. The electrolysis cells, according to the vertical technique, were equipped with insoluble titanium anodes, coated with IrO2. In the cells was a zinc sulphate electrolyte with a pH value of 1.5 and a temperature of 55 C. Each of the four anodes, two for the upper side and two for the lower side of the strip, were supplied with an electrical current of 3000 A. With a speed of the strip of 30 meters per minute, a layer of zinc having a thickness of approximately 2 micrometers was deposited. After the electrolysis cells, the strip was rinsed with water and finally dried. The product obtained had a surface

regular, with silver reflection.

  In subsequent tests, good adhesion of the layer was found

  deposited on the surface of the strip.

  Test N 2: During this test, the strip was pickled with hydrochloric acid with a concentration of 16%, at a temperature of 80 C. The strip was then introduced, without passing through an intermediate rinsing, into electrolysis cells. These cells were fitted with soluble zinc anodes in the form of bands. The electrolyte used was a zinc chloride-based electrolyte having a pH value equal to 3. Despite an iron content of approximately 5 g / l, a layer of pure zinc was deposited. The surface of the coated strip was even lighter than that resulting from the deposition from the zinc sulfate electrolyte. The adhesion of the deposited zinc layer has been found to be very good. Metallographic examinations have also shown that the pickling process

  had been fully active and no residue of carbon adhered to the strip.

  Test N 3 During this test, the solution leaving the pickling unit which then used hydrochloric acid and which had the following approximate composition, namely g / l of Fe, 60 g / l HCl and 190 g / l of total chloride, was pumped through a unit of

  dissolution in which zinc granules were found.

  After the dissolution process, the rest of the hydrochloric acid was used to dissolve the zinc. The outgoing solution then had the following composition: Fe 100 g / l, Zn 54 g / l, chloride 190 g / 1. This solution was introduced into the electrolysis unit as an electrolyte. The electrolysis cells were equipped with insoluble anodes and a diaphragm. As a result of the subsequent application of an electric current, a layer of zinc-iron was deposited on the strip. Analysis of the iron content of the deposited layer gave a content of between 11 and 13%. During the continuation of the test, the solution was extracted from the electrolysis cells and introduced into the pickling unit. It was found that the free acid formed in the electrolysis unit could

  be used in the pickling unit to dissolve scale.

Claims (8)

  1. A method of manufacturing a hot-rolled strip coated by electrodeposition, characterized in that the hot-rolled strip is passed continuously online, through a chemical pickling unit (2) and then through a electroplating unit (10) comprising electrolysis cells. 2. Method according to claim 1 characterized in that the strip is rinsed between   pickling and plating units.   3. Method according to any one of claims 1 or 2 characterized in that   that hydrochloric acid is used as pickling liquid and, as   plating liquid, a chloride-based electrolyte.   4. Method according to any one of claims 1 or 2 characterized in that   that we use, as pickling liquid, sulfuric acid and, as liquid   electroplating, a sulfate-based electrolyte.   5. Method according to any one of claims 1 to 4 characterized in that   use the solution formed during pickling, containing iron, as liquid for plating.   6. Method according to any one of claims 1 to 5 characterized in that   that is used, as a pickling solution, in at least one stage of the   pickling, the solution leaving the electrolysis cells.   7. Installation for manufacturing a hot rolled strip coated by electrodeposition, characterized in that a chemical pickling unit (2) is connected in   line with an electroplating unit comprising electrolysis cells (10).   8. Installation according to claim 7 characterized in that the chemical pickling unit (2) and the electrolysis cells (10) are separated by a rinse (9).