FR2503740A1 - PREPARATION AND PROTECTION OF STAINLESS STEEL SURFACES IN THE MANUFACTURE OF HOT-ROLLED SHEET - Google Patents

Abstract

The invention relates to a method of protecting stainless steels which are applied before the production of the hot-rolled sandwich. A metal powder, for example nickel, is dispersed in ethyl alcohol in the form of particles having a compact configuration and is deposited electrophoretically on the stainless steel. The resultant deposit has beneficial adhesion without the necessity of a further post-treatment.

Description

 The present invention relates to a technique for the preparation and protection of stainless steel surfaces in the manufacture of plated t81e by hot rolling, in which the two materials (structural steel and stainless steel thus treated) are rolled together, after have been assembled into a "sandwich".

 The present invention is characterized in that a metal layer, deposited by electrophoresis from a suspension in which it is contained in dispersion in the form of particles preferably having a compact structure, is interposed between the two materials.

 It is a well-known fact that with conventional plating techniques by hot rolling, there are in particular great difficulties in avoiding the oxidation of stainless steel, which results in a poor union between the materials or makes their union completely impossible. This difficulty is further aggravated by the different thermal expansion of the constituent materials, which means that great care must be taken in the preparation of the sandwich intended for rolling.

 In the current state of the art, this problem is solved by eliminating as much air as possible from the place where the rolling is to be carried out, by adopting a reducing atmosphere in the "sandwich" or by coating the stainless steel (with electrolytic methods, by welding or by explosive charge).

 the present invention opens a less coating and sometimes better way for the deposition of the protective layer, in comparison with the various conventional coating methods applied previously.

The technique which is the subject of the present invention is characterized by a combination of the following operations:
- Preparation of a suspension of metal particles in absolute ethyl alcohol, in the form of particles having a compact structure (as can be obtained for example by grinding the powder with the dispersion medium, by known methods which are generally considered to reduce the tendency of particles to take a flat shape).

 - Installation of the stainless steel sheet as the cathode of a special electrophoretic tank, into which the above suspension is introduced continuously, so as to obtain a surface deposit.

 It has been found that good results are obtained when the suspended metal particles are made of nickel and / or nickel alloys. the suspended particles can also be made of metal oxides: in this case, electrophoretic deposition requires post-treatment in a reducing atmosphere, for example of hydrogen in nitrogen, at a temperature of 7500C and above.

 The technique, which can be described as cataphoretic (that is to say an electrophoretic deposition at the cathode), also includes the addition, to the suspension, of a non-contaminating agent (for example based on ferric ions ) capable of charging the particles by forming what is called the "double electric layer" according to a well-known mechanism.

 It has been observed that if the suspension according to the invention is used and if the operating conditions which are suitable are applied, a compact adherent deposit is obtained which has all the characteristics required for the particular application considered and which, consequently, requires no further treatment.

 This result should be considered unexpected, having regard to the fact that the general opinion, even of authoritative scientific personalities, is that electrophoretic techniques can only give adherent and compact coatings if the deposited substance has inherently du- pro - adhesive properties (for example painting). On the other hand, inorganic powders, binders have so far been added to the suspension which it is desired to deposit, in order to provide the cataphoretic coating with the desired characteristics. In all cases, the nature of these binders (organic or inorganic depending on the circumstances) was suitable for discouraging their use in the particular technique to which the present invention relates.

The reason is that c, r and Si would produce deleterious effects on the adhesion of the materials used for plating and on their interface properties, as a result of the formation of gaseous substances or the alteration of the surface composition. Consequently, in the particular case, the state of the art concerning cataphoresis teaches that, for the deposition of metal powders, it was essential either to add special binders or to use post- treatment involving sintering, compacting, an alloy or the like. However, contrary to the prior indications of the technique, it has been discovered with a certain surprise that the operating conditions developed in connection with the process of the invention make such additional treatment perfectly unnecessary.

 The present invention also provides an embodiment which improves the protective effect of the deposit. It consists in using a sheet (of metal or alloy) less than 2 mm thick, which is applied to the electrophoretic deposit by welding the edges without the use of weld metal, for example by adopting the TIG technique (tungsten - inert gas). The chemical nature of the sheet, as well as the underlying intermediate deposit, can determine the particular characteristics of the product, in the intermediate layer between the support and the plating material.

 The present invention is not limited to the process for depositing the protective material: it extends also encompassing a device which has proved to be extremely functional.

 This device is shown by way of example in FIGS. 1 and 2. FIG. 1 is an overall perspective view of the electrophoresis tank, while FIG. 2 shows more particularly the mobile device which carries the counterelectrode ( anode). The suspension contained in the funnel-shaped container S enters the parallelepipedic tank 1 which is supported by the base 2 and is reinforced by props 3 and by belts 4. A pump V1, which may for example be of the peristaltic type, drives the suspension S in the tank where the stainless steel plate L is arranged in such a way that its face opposite to that which must be coated is near one of the walls. The suspension flows on the surface of the plate X, then leaves the tank via the overflow T and is collected in the container S, from the bottom of which the suspension is recycled to the tank. This container shape ensures sufficient agitation of the suspension. Figure 1 also shows the DC power supply.

 During the electrophoretic deposition, the counter-electrode consists of a metal sheet 5 which has approximately the same width as the plate to be coated and which is supported by the mobile device 6, capable of sliding up and down in the seat formed by two guides inside the tank. The device is equipped with a suction pipe 7, connected to a pump V2, for recovering the suspension which exceeds its upper level.

 The plate which must be coated during the electrophoretic treatment remains fixed, resting on the V-shaped bottom of the tank 1 (indicated by broken lines).

 On the other hand, the counter electrode (anode) is constantly in motion, at a speed of the order of 50 cm / min, during the treatment.

 the mobile device is connected by u; collar 8 to a guide 9 fixed to the upper part of the tank and its functions are as follows - Maintain the counter-electrode at a constant fixed distance from the plate to be coated.

- Maintain the level of the suspension at a constant distance from the counter-electrode (this is done by aspiration of the suspension which, as the mobile device gradually descends towards the bottom, overflows into the device and collects in the V section.

More generally, the apparatus according to the invention, for the electrophoretic deposition of a protective coating on stainless steels in the manufacture of plated materials, essentially consists of the following elements
- A reservoir of non-conductive material, having essentially a parallelepiped shape, in which the plate to be coated is placed near one of the walls where it serves as a cathode.

 - A device for the continuous introduction, from the bottom, of a suspension of the substance to be deposited and a device for discharging the excess suspension.

 - A metal counter-electrode, having substantially the same length as the width of the plate, supported by a mobile device which moves in the interval between the plate and the non-neighboring wall, parallel to the latter, at a distance fixed constant.

 - A suction line, ending at the lowest point inside the mobile device, to evacuate by suction the suspension which enters the device while it is moving up and down, thus ensuring that the level of the suspension will be kept at a constant distance from the counter electrode.

 If the height of the plate to be coated is more than one meter or if the suspension is not sufficiently stable, the latter can be introduced by spray nozzles attached to the mobile device at a fixed distance below the counter-electrode. . The spray jets ensure a better distribution of the suspension over the entire width, as well as greater homogeneity of the suspension during the treatment.

 The same effect can also be obtained to a certain extent by reversing the direction of circulation of the suspension in the two pumps V1 and V2 indicated in the explanatory diagram.

 a general description of the invention has so far been given. A more detailed description of the objects, features and advantages of the invention is now offered, with the aid of the following examples - which are offered by way of explanation and should in no way be interpreted as restrictive.

EEMmJ3 i.-
The entire surface of an AISI 405 ferric stainless steel plate, measuring 120 x 210 x 20 mm, was cleaned by shot blasting. Then the plate was coated, in accordance with the present invention, in the electrophoretic tank which is the subject of the invention, supplied with a suspension of 20 g / l of nickel powder in absolute ethyl alcohol. The electrical energy was delivered at 400 V per cm of distance between the electrodes.

An adherent and compact nickel deposit, some 15 microns thick, was obtained.

 the surface of an AISI 304 austenitic steel plate, measuring 120 x 210 x 16 mm, was subjected to exactly the same treatment.

 The two resulting plates were applied separately to two panels cut from 90 mm thick carbon steel and welded directly with two bordering beads, using electrodes with a high Ni content.

 the asymmetric sandwiches obtained in this way were heated to 11800C and 12200C and rolled in a three-stage rolling mill, from a total initial thickness of 110 and 106mm to a final thickness of 25mm in eight passes.

 Each sandwich produced a plated plate which, upon ultrasound examination, was found to be perfectly healthy.

 It was found that the connection at the interface between the base material and the coated material was hopelessly good, with respective values of 39 kg / mm2 and 34 kg / mm2.

 The conditions at the carbon steel / AISI 405 steel interface of the first of the two plated plates are illustrated by the photomicrograph of FIG. 3 (x320), obtained with a scanning electron microscope and indicating the chromium concentration profiles ( S curve) and nickel (curve with crest).

EXAMPLE 2.
By proceeding under the same conditions as in Example 1 and using a piece of AISI 304 steel sheet of the same dimensions as that of Example 1, but with a suspension composed of 20 g / l of oxide of nickel in absolute alcohol and with a direct current supply of 400 V per centimeter of distance between the two plates, a homogeneous, compact and adherent coating was obtained. This coating was reduced by mixing 10% R2 in nitrogen at 8000C and the coated sheet was used to make an asymmetric sandwich which was hot rolled in the same way as in Example 1. Ultrasonic examination showed that the resulting plated material was sound.

 Shear tests, carried out on the clad sheet, gave a value of 32 kg / mm2.

EXAMPLE 7.
Two AISI 304 steel plates 15 mm thick, with sanded surfaces and measuring approximately 1 m2, were coated on one of their faces in accordance with the invention, a layer of nickel being deposited by electrophoresis from of a suspension in absolute ethyl alcohol. A decarburized steel sheet, 0.7 mm thick, was applied to one of the coated plates, then was welded along the edges without the use of weld metal. the two plates thus prepared, one coated with nickel and the other coated with nickel and then protected by the sheet, were used to prepare an almost symmetrical "sandwich" according to FIG. 4. This is a schematic sectional view materials assembled in a "sandwich" according to this example. A and B are the basic materials and a and b the plating materials. the electrophoretic Ni coating is designated by c, while the protective sheet applied to the plate b is designated by d.

 A layer of Cr 2 O 3 separating agent was applied to the two untreated surfaces e (by a known technique), in order to facilitate the separation following hot rolling. The surfaces were placed face to face, then welded along the edges with a continuous weld bead. the "sandwich" was closed by the insertion of four pieces of carbon steel designated by f, then welded along the edges by application of a known technique.

 After heating to 12200C in a crucible oven, the "sandwich" was rolled in a four-stage inversion rolling mill, from a total initial thickness of 250 mm to a final thickness of 50 mm.

 After separation, the two plated plates were found to be healthy on ultrasound examination of the entire surface.

Shear tests have given respectively. results in 2 states of 39 and 32 kg / mm2.

Claims (7)

- CLAIMS  1.- Method for depositing a protective coating on stainless steels in the manufacture of plated materials by hot rolling, characterized by a combination of the following operations - Preparation of a suspension composed of metal powders and / or oxide of metal in dispersion in absolute ethyl alcohol, in the form of electrically charged particles; - Installation of the stainless steel plate as a cathode in a special electrophortic tank, into which the above suspension is introduced continuously, so as to obtain a deposit.  2.- Method according to claim 1, characterized in that-the suspension is prepared by grinding the powder with the dispersion medium and in that the particles in suspension are loaded with a non-contaminating agent.  3.- Method according to claim 2, characterized in that the suspension contains, as metal particles, nickel and / or nickel alloys and / or nickel oxides.  4.- Method according to claim 3, characterized in that when the deposit is obtained from suspensions which also contain oxides, arrangements are made for a post-treatment of reduction in the metallic state.  5.- Method according to claim 4, characterized in that a metal or alloy sheet, preferably having a thickness between 0.2 and 2 mm, is applied to the metal deposit.  6.- Device for electrophoretic deposition; - a protective layer on stainless steels in the manufacture of plated materials, characterized in that it comprises  - A reservoir (1) of non-conductive material, essentially having a parallelepiped shape, in which the plate to be coated (B) is placed near one of the walls, where it acts as a cathode.  - A device for the continuous introduction, from the bottom of the tank, of a suspension of the substance to be deposited and for the pouring of the excess suspension.  - A metal counter-electrode (5), the length of which is practically equal to the width of the plate (L), supported by a mobile device (9? Which moves in the interval between the plate and the non-neighboring wall parallel to it, at a fixed constant distance.  - A suction pipe (7) in the mobile device for sucking, from the lowest point thereof, the suspension which enters it as a result of the overflow during its movement from top to bottom, which guarantees in the main thing is that the level of the suspension will remain at a constant distance from the counter electrode.  7.- Method according to any one of claims 1 to 5, characterized in that the electrophoretic deposit is obtained by placing the stainless steel plate (l) as a cathode in an electrophoresis tank, this plate remaining fixed and resting on the bottom of the tank, while the counterelectrode (anode 5) is moved up and down during the coating process.