WO1993018869A1

WO1993018869A1 - Method and device for reducing scale formation during hot forming of metal, in particular steel

Info

Publication number: WO1993018869A1
Application number: PCT/EP1993/000545
Authority: WO
Inventors: Klaus GRÜTZMACHER; Vladimir JÄGER; Friedrich Raffauf; Klaus Ulrich
Original assignee: Klöckner Stahl Gmbh
Priority date: 1992-03-14
Filing date: 1993-03-10
Publication date: 1993-09-30
Also published as: ZA931765B; EP0630295A1; AU3632293A; PL170068B1; ES2088276T3; DE4208208A1; CN1078179A; TW227019B; EP0630295B1; DE59302830D1; DE4208208C2

Abstract

The invention concerns a process for reducing scale formation during hot forming of metal, in particular steel. The metal, which is formed in its heated state and under brief high pressure, the deformation being substantially plastic, is subjected during a subsequent stress-relief phase, during which the degree of plastic deformation is reduced, to treatment with an inert gas such as nitrogen. In a device for carrying out the process, a device for supplying inert gas is connected to the device used for hot forming the metal; the former has a large number of jets directed towards the formed metal as well as a system for restricting the supply of inert gas, after the metal has been formed, to the stress-relief phase. In the hot-rolling of steel, for example, inert gas jets are arranged at the output end of a roll stand in such a way that the inert gas is blown obliquely against the direction of feed of the steel into the space created around the formed metal between the output end sections of the last roll housing and the open espaces of the roll gap at the sides of the formed metal. An inert gas pressure higher than the atmospheric pressure thus builds up in the said space.

Description

Method and device for reducing scale formation during hot forming of metal, in particular steel

Description

The invention relates to a method and a device for reducing scale formation during hot forming of metal, in particular steel, in which the heated metal is briefly elastically deformed under high pressure and can then relax.

All base metal reacts on the surface with constituents of the atmosphere surrounding it and forms a corrosion layer through scaling, which protects the underlying metal from further corrosion. In air, their oxygen reacts with the metal and forms a scale layer from the metal oxide. This oxide layer is brittle. If the metal is severely deformed, it tears and jumps off slightly, but forms again under the influence of the surrounding atmosphere.

In the hot forming processes mentioned at the outset, it was observed that fine scale particles detach themselves from the metal surface to a considerable extent and escape as suspended matter in the form of aerosol clouds. For example, on hot rolling mills, significant emissions of iron oxide and (corresponding to the alloy components) heavy metal particles occur behind each roll gap. Such emissions represent a considerable burden on the environment and, above all, on those working in the rolling mill. Until now, attempts have been made to master these emissions by suction. The equipment required for this, which in particular requires filtering out the suspended matter particles from the air or the cooling water for the rollers, is considerable; considerable amounts of steel are also lost.

It has now been found that, surprisingly, scale formation can be decisively reduced if the surface of the metal is covered with inert gas in accordance with the invention in the relaxation phase. The reason for this has not yet been clearly clarified, it is not immediately obvious.

It can be assumed that at the high temperatures to which the metal has to be heated in order to achieve a ductile state, the chemical reactions of scale formation accelerate, indeed occur spontaneously. It can also be assumed that the scale layer on the metal at the start of a hot forming process tears during the deformation and in any case largely detaches. A closer look and evaluation showed that the strong scale emissions, such as occur when a heated steel strip passes through a roll stand, apparently cannot be caused solely by the scale layer which is present on the metal surface at the start of the forming process.

The fact that a new scale layer forms quickly on the deformed metal leads to the interpretation that in the relaxation phase of the deformed metal a certain number of scale formations and detachments occur in a cascade-like manner, because plastic deformation also occurs Elastic deformation is associated with this and this leads to a certain re-expansion of the metal, which may lead to the formation, tearing and cracking of scale layers in rapid succession.

So far, however, there is no certainty for this course of events; the only thing that is certain is that, contrary to technical expectations, covering the surface of the hot-formed metal for a certain period immediately after the hot deformation - in hot rolling material this corresponds to a certain distance after leaving the nip - to a drastic reduction in the Scale formation (more precisely: the formation of clouds from scale particles detached in the form of suspended matter) leads.

Nitrogen is preferably used as the inert gas, with which the material leaving the roll gap is blown over a certain distance during the hot rolling of steel.

In order to cover the entire critical area in such a case, during the hot rolling of steel after When leaving the nip the formation of aerosol clouds from scale and heavy metal particles occurs, when steel is hot rolled the steel leaving the nip is blown with the inert gas over a certain distance. This distance corresponds in the context of starting vorste¬ given interpretation of the 'relaxation phase ^• Stahl¬ the strip after its plastic deformation in the roll gap; in any case, it has been shown that, after this particular distance, there is naturally a (normally thin) regeneration, but no longer the emission of those suspended matter.

In this application of the method according to the invention, the blowing direction of the inert gas is preferably set obliquely against the feed direction of the steel. In a further development it can be provided that the inert gas is also blown into the free spaces of the roll gap lying to the side of the rolling stock.

The covering according to the invention of the heated metal which is subjected to a hot forming process during its relaxation phase after the plastic deformation has ended is expediently carried out in any case by blowing with inert gas even when the forming process, such as in open die forging, drop forging or hot pressing, essentially ¬ Lichen is stationary and therefore the space surrounding the workpiece and the tools can be shielded from the air atmosphere in a relatively simple manner. Accordingly, a device for carrying out the method according to the invention is that a device for supplying inert gas is connected to a plant for hot forming of metal, which device has a plurality of nozzles directed against the material to be formed and means which prevent the supply of inert gas after the forming the relaxation limit the phase of the application. It is preferably provided that the device has elements which at least enclose the material to be deformed during the relaxation phase, and on the inside of which the inert gas, which is at a higher than atmospheric pressure, can be supported. It has been shown that a certain inert gas pressure is required in order to bring it sufficiently close and intensively to its surface despite its spontaneous heating when the material to be formed is approached. However, such pressure of the inert gas cannot be built up without at least largely enclosing the material to be formed during the expansion phase.

Insofar as the material to be formed is rolled steel which passes the roll stands at high (and increasing) speed, the enclosing elements can be parts on the output side of the roll stand of a roll stand.

The invention is explained below using the drawings using the example of a hot strip mill stand. In the drawings:

1 shows a schematic representation of the part of the roll stand on the output side;

2 the top cooling water tank in a top view;

Figure 3 shows the lower roller table in plan view.

Figure 4 is an illustration of the U-shape process in the roll gap. and

5 shows a vertical central section through the rolls and the rolling stock in FIG. 4.

Of the work rolls 2, 3 and support rolls 4, 5 held in the stand 1 of a roll stand, only sections shown in a schematic side view. The heated rolling stock passes through the roll gap 6 from left to right in FIG. 1; the elements belonging to the rolling stand shown in FIG. 1 are therefore located on the output side of the rolling stock.

These elements include, below the hot strip 7, a roller table consisting of the two parts 8a and 8b with a stripper 9 resting on the work roll 3, and a cooling water distributor 10 below the roll table 8a. The cooling water delivered by these to the work roll 3 is at 11 indicated.

Above the hot strip 7, a water tank 12 can be seen in FIG. 1, the underside 16 of which is provided with a stripper 13 in the area of the work roll 2. Distributors 14 for the cooling water 15, which is sprayed onto the work roll 2, are arranged in the water box 12. The scraper 13 prevents the cooling water from draining onto the hot strip 7; the cooling water runs off to the side in the region of the cheeks of the stand 1 and is discharged below the roll stand.

According to the invention, both in the area of the roller table 8 and the upper water tank 12, devices for blowing nitrogen into the hot from the table and the water tank with their wipers 9, 13 and the lateral components of the stand 1 are largely enclosed Hot strip 7 is provided immediately after leaving the roll gap 6 through the space. A pipe 20 running transversely to the rolling direction leads through a hose line 21 to a double pipe transverse distributor 22, which distributes it to a plurality of relatively thin nozzle pipes 23, which have their openings 24 push through the underside 16 of the water tank 12 in a sealed manner and direct an inert gas jet 25 at an angle against the direction of advance of the hot strip 7 against the hot strip 7.

Another hose connection 26 leads from the inert gas tube 20 to a transverse distributor 27 arranged in the feed direction of the hot strip behind the water tank 12, which is provided with various nozzle openings 28 distributed over the width of the hot strip 7 and in turn is positioned at an angle to the inert gas jets against the feed direction of the hot strip 7 29 issues. The same applies at an even greater distance from the roll gap 6 for the transverse distributor 30, which is fed from the pipe 20 through a distributor pipe 31.

In a corresponding manner, the underside of the hot strip 7 is covered with inert gas. From a tube 40, a double tube transverse distributor 42 receives the inert gas via a tube 41 and distributes it to a plurality of nozzle tubes 43 distributed over the width of the roll stand on nozzles 44 which pass through the lower roller table 8a and are inertly inclined against the feed direction of the hot strip 7 ¬ direct gas jets 45 against it.

By means of further hoses 46, 47, cross distributors 48, 49 are connected to the pipe 40 and direct inert gas jets 50, 51 (again at an angle to the feed direction) (through the spaced-apart individual elements of the roller table 8b; see FIG. 3) against the Bottom of the hot strip 7.

The passage of the hot strip 7 through the work rolls 2, 3 is illustrated schematically in FIG. 4. The warm band 7 comes from the left with the initial thickness s and is plastically deformed in the roll gap 6 to a smaller thickness, which has the size s _χ at some distance from the roll gap 6. The scale layer 7a, which surrounds the hot strip 7 on all sides before entering the roll gap 6, can be seen in FIG. 4 to the left of the roll gap. This scale layer is torn open and blown off in the course of the wedge-shaped deformation; in Fig. 4 this is indicated greatly enlarged.

As soon as the hot strip 7 has passed the roll gap (ie the line with the smallest distance between the work rolls 2 and 3), the distance between the rolls 2 and 3 increases rapidly, so that the material of the hot strip 7 can relax. Depending on the material properties and the feed speed of the hot strip 7, the zone E of the relaxation phase extends over a certain distance which adjoins the roll gap 6 in the feed direction and which is identified by “E” in FIG. 4 on the top and bottom of the hot strip 7. In area E of this relaxation phase, the hot strip 7 is covered according to the invention with inert gas, preferably nitrogen, so that no new scale can form there on the strip. As soon as the hot strip 7 leaves the relaxation zone E covered with inert gas, the metal surface reacts with the oxygen in the ambient air and forms a (new) backing layer 7b of comparatively small thickness.

5 shows a schematic vertical section through the work rolls 2 and 3 and the hot strip 7 located in the roll gap 6. According to a further development of the invention, the inert gas is also blown into the free spaces 52 of the roll gap 6 located next to the hot strip 7.

Claims

EXPECTATIONS

1. A process for reducing scale formation during hot forming of metal, in particular steel, in which the heated metal is plastically deformed briefly under high pressure and can then relax, characterized in that the surface of the metal is covered with inert gas in the relaxation phase.

2. The method according to claim 1, characterized in that nitrogen is used as the inert gas.

3. The method according to claim 1 or 2 in the hot rolling of steel,

characterized in that the steel leaving the roll gap is blown with the inert gas over a certain distance.

4. The method according to claim 3, characterized in that the blowing direction of the Inertga¬ ses is set obliquely against the feed direction of the steel.

5. The method according to claim 3 or 4, characterized in that the inert gas is also blown into the free spaces of the roll gap lying laterally next to the rolling stock.

6. Device for performing the method according to at least one of claims 1 to 5 on a system for Hot forming of metal, characterized in that a device for supplying inert gas is connected to the plant for hot forming, which device has a plurality of nozzles directed against the material to be formed and means for supplying inert gas after the forming to the relaxation phase restrict.

7. The device according to claim 6, characterized in that the device has elements which enclose the material to be formed at least during the relaxation phase, and on the inside of which the inert gas, which is at a higher than atmospheric pressure, can be supported.

8. The device according to claim 7 for carrying out the procedural method according to claim 4 or 5, characterized in that the elements are output-side parts of the roll stand of a roll stand.

CHANGED CLAIMS

[at the international office on 3 August 1993 (03.08.93) received original claims 2, 4 and 8 deleted; Claims 1 and 3 replaced by amended claim 1; Claims 5, 6 and 7 replaced by amended Claims 2, 3 and 4 (2 pages)]

1. A method for reducing scale formation during hot rolling of metal, in particular steel, in which the heated metal is plastically deformed briefly under high pressure with an elastic component and can then relax, the rolling stock being covered on the outlet side with inert gas, characterized in that the rolling stock leaving the roll gap is blown with the inert gas over the distance within which the relaxation of the elastic deformation component takes place.

2. The method according to claim 1, characterized in that the inert gas is also blown into the free spaces of the roll gap lying to the side of the rolling stock. 3. Device for performing the method according to claim 1 or 2 on a roll stand for hot rolling metal, in which a device for supplying inert gas is connected to the roll stand, which has a plurality of nozzles directed against the material to be formed, characterized in that means are provided which extend and restrict the supply of inert gas after hot rolling to the distance to the roll gap corresponding to the elastic relaxation phase of the rolling stock.

4. The device according to claim 3, characterized in that the means contain housing elements in which the inert gas can build up to a higher than atmospheric pressure.