EP0834591A1 - Process and apparatus for hot-dip coating of steel sheets - Google Patents

Abstract

Sheets or a continuous band of steel are coated with metal by being passed continuously from an annealing furnace into a bath of the molten metal through a metal duct whose lower end dips into the bath, the lower part of the duct being controllably heated to around the temperature of the bath. Also claimed is a vessel containing a bath of molten metal with a metal duct dipping into it and means to guide steel sheet through the duct and into the bath, wherein at least the lower part of the duct level with the bath surface is double-walled to accommodate controllable heaters.

Description

OBJECT OF THE INVENTION

The present invention relates to a method of coating of steel sheets by immersion in a liquid metal in which the sheets pass through continues, an annealing oven and descend into the bath liquid metal through a metallic sheath extending between the oven and the bath and whose lower end plunges into the bath. The invention also relates to a device for implementing this method.

TECHNOLOGICAL BACKGROUND

It is well known that the coating of sheets of steel by immersion in a bath of liquid metal causes appearance defects in the finished product which can be extremely annoying, even prohibitive for the use of the product. Among these faults are those which are linked to the presence of so-called solid impurities "intermetallic" in the bath and which result from the dissolution of the sheet in the already saturated liquid bath in iron.

These defects are well known in the case of zinc and aluminum coating (galvanization and aluminized) and also appear for their alloys such as aluzinc and aluminized with silicon.

Numerous studies have shown that the intermetals in question have a size ranging from 2 to 50 microns and are made up of iron and alloy metals. These are in particular the delta phase (FeZn) and Fe ₂ Al _5-x Z _x in galvanization, Fe ₅ Al ₂₀ Si ₂ in aluzinc, Fe ₂ Al ₉ Si ₂ in aluminized with silicon and FeAl ₃ in Galfan and pure aluminized.

Significant efforts have been made to eliminate these defects and a significant number of patents have have been filed concerning the shape of the coating tank, control of the atmospheres in the oven and in the sheath, the shape of the sheath surrounding the sheet until it enters in the bath, the surface cleaning systems of the bath, etc. Unfortunately, none of the known proposals was not entirely satisfactory.

DESCRIPTION OF THE STATE OF THE ART

In document JP 63 111162 A (SUMITOMO IND LTD), a device for controlling the temperature is described total bath, particularly in the area of the submerged tube.

The heating power therefore remains significant same as power consumption.

Document JP 08 176773 A (SUMITOMO METAL IND LTD), describes a device for controlling the temperature relating to the condensation of vapors in the submerged tube.

PURPOSE OF THE INVENTION

The object of the present invention is to provide a new method and device for reducing significantly the formation of these defects.

CHARACTERISTIC ELEMENTS OF THE INVENTION

To achieve this objective, the process according to present invention is characterized in that one heats the sheath, in a controlled manner, so that its temperature of skin, at the level of the mooring line, and in particular inside, be kept at a temperature roughly corresponding to that of the metal bath.

The temperature of the sheath and, in particular, of his skin, side exposed to the sheet, is advantageously controlled in an area between 3 cm above and 3 cm below the mooring line so that t - 30 ° C <T <t + 50 ° C, where t is the bath temperature and T the temperature of the sheath.

The invention also provides a device for coating of steel sheets by immersion in a bath of liquid metal comprising a tank containing the metal liquid, a metallic sheath whose lower end plunges into the metal bath and means to guide the sheet metal through the sheath in the metal bath, characterized in that the sheath comprises, at least at the level of the bath surface, a double wall inside which there is a thermal loss compensation system, for example a heating system placed under control a device for regulating the skin temperature of the inner surface of the sheath.

The heating system can consist of electrical resistors while the regulation system may consist of thermocouples.

This heating system can, moreover, be associated with a thermal insulation system of the sheath.

The invention is based on observations and measures relating to the formation of these faults and essentially consists in eliminating parameters and conditions favorable to the formation of defaults.

It has, in fact, been observed that the cause essential to the formation of these defects is the unhooking of intermetallics which have crystallized previously on the "cold" parts of the sheath and which then come off to attach to the sheet and are driven by them.

Observations made on an industrial line have shown that the situation is particularly critical where the annealed sheet enters the metal bath at inside the sheath, this because of the physicochemical state from its surface if these particles are in contact with the sheet metal there, that is to say before the galvanizing reaction does not develop, the experiment has shown that it was practically impossible for them to drop out naturally. On the contrary, the contact of the sheet metal on the submerged deflection rollers reinforces their grip on this sheet.

The origin of these intermetallics is a limited solubility of iron in the bath while this element is continuously supplied by the sheet itself. Indeed, when this sheet enters the bath, part of the surface of the sheet is dissolved by the liquid metal that is already saturated with iron. This is here observations whose reasons are not yet known fully explained with certainty. It therefore results temporary iron supersaturation of the bath leading to precipitation of intermetallic compounds with elements of the bath, where thermodynamic conditions are the most favorable.

These compounds remain in suspension when are small enough or decant, depending on their relative density, either towards the surface or towards the bottom when they have reached a sufficient diameter.

Observations have shown that intermetallics form and crystallize more especially on surfaces colder than the bath. This is explained physically by the fact that at the extreme near these colder surfaces, the temperature of the bath is also lower and therefore the solubility of iron is reduced there. The germination conditions are therefore done, favored.

These surfaces or cold spots are between others on the sheath, generally in stainless steel, and more particularly at the bath surface. Of online thermal and thermographic readings have shown that the temperature gradient existing in the emerged part of a room, neither heated, neither isolated, and which soaks in the metal bath, is huge. We thus found in a galvanizing bath at a temperature of 460 ° C than the temperature of a room stainless steel 70 mm thick was only 390 ° C at 1 cm above the mooring line and 235 ° C at 10 cm above this same line. This is so of the sheath at the mooring line. This results in a preferential crystallization of intermetallics at this place.

Under normal industrial conditions, the sheath is subject to variations in thermal regimes as a result of inevitable variations in the level of bath surface, either by waves or by adding fresh metal (liquid or solid). When, the line of anchorage is displaced, for example following a rise in the bath level, the temperature profile at this point is amended. In particular, cooler areas can be brought back to bath temperature. He was noticed that these conditions are favorable for dropping out of previously crystallized particles. The mechanism exact chemical is not known but could, for example, result from a partial solution or a transformation of the chemical composition. If, moreover, the particle thus detached is of lower density than the bath, it floats and can be attracted, trained and fixed on the sheet scrolling in its vicinity and thus create the appearance defects.

So these are the fluctuations of the thermal regime sheath at the mooring line which successively promote crystallization and particle dropout and defect generation if this stall takes place inside the sheath connecting the metal bath oven. It is also well known that variations in the surface of the bath, as they occur when loading zinc ingots or cleaning the bath, generates a large amount of faults appearance.

It is therefore thanks to thermal insulation particularly neat or, more industrially, to the controlled heating of the sheath wall that the invention succeeded in removing cold spots and as soon at the origin of the crystallization of intermetallics on these cold parts. Their untimely stalling during inevitable variations in the level of the bath cannot, from then, more happen.

The attached figure shows schematically, at illustrative title, an example of an embodiment of a device according to the present invention.

The figure shows a sheet metal strip 2 in from an annealing oven not shown entering a liquid metal bath 4 for coating thereon, by example by galvanizing. Between the oven and the bath 4, the sheet metal passes through a closed sheath 6 of which only the lower part which plunges into the metal bath 4.

In accordance with the present invention, the sheath 6 includes, at least in the part that plunges into the bath 4, a double wall 8 between which is a system for heating, for example electric resistors 10 placed under the control of a regulation system including a skin temperature measurement, for example a thermocouple 12.

Resistors 10 and the regulation system are designed to ensure, permanently, in the area between 3 cm above and 3 cm below the level of bath 4 and in particular at the level of the line of wetting, a skin temperature inside the sheath 6 which is in the range between t - 30 ° C and t + 50 ° C, t being the temperature of the metal bath 4.

Heating and thermal insulation of the duct 6 can advantageously complement each other without their association is compulsory, thermal insulation can be carried out by conventional insulators or by ceramic coatings.

In this way, a thermal regime is ensured sheath constant 6 at the mooring line and we avoid crystallization and dropout of particles which are responsible for the appearance defects of coated sheets.

Claims (6)

Method of coating steel sheets with immersion in a bath of liquid metal according to which the sheets continuously pass through an annealing oven and descend into the liquid metal bath (4) through a metal sheath (6) extending between the oven and the bath (4) and whose lower end plunges into the bath (4), characterized in that the sheath (6) is heated so controlled so that its skin temperature, at the level of the mooring line and, in particular, inside, either maintained at a temperature corresponding approximately to that of the metal bath (4). Method according to claim 1, characterized in what we set the temperature T of the sheath in an area between 3 cm above and 3 cm below the mooring line so that t - 30 ° C <T <t + 50 ° C, where t is the bath temperature. Device for coating steel sheets (2) by immersion in a bath of liquid metal comprising a tank containing the liquid metal (4), a sheath metal (6) whose lower end plunges into the metal bath (4) and means for guiding the sheet (2) to through the sheath (6) in the metal bath (4), characterized in that the sheath (6) comprises, at least at the level of the bath surface, a double wall (8) inside which is a heating system placed under the control of a temperature regulation system skin of the inner surface of the sheath (6). Device according to claim 3, characterized in that the heating system consists of electrical resistors (10). Device according to claim 3, characterized in that the regulatory system consists of measures skin temperature and thermocouples or others. Device according to any one of Claims 3 to 5, characterized in that the system for heating of the wall (8) of the sheath (6) is associated with a thermal insulation system.