Production of metal strip

The invention relates to a method and an apparatus for the continuous production of a

Metal strip, which method comprises compacting powder to form a green strip, inserting the strip into a sintering furnace, and supporting the strip with a gas cushion during its course

Transportes through the sintering furnace covers.

Methods for the continuous production of metal strips and other forms by compacting powder are already known. In these methods, cold compaction alone is not sufficient to achieve a strip of density and strength as achieved by a strip rolled from an ingot. The compacted powder must be sintered, ie to a temperature at which the powder tends to bond (by incipient melting or by diffusion at high temperature and solid state). The sintering may be followed by further densifications and heat treatments to obtain a strip having suitable mechanical properties and surface finish. In this respect, the object is to produce a strip of sufficient density and with mechanical properties that are comparable to the properties of those strips that are rolled on an ingot.

The strips should be best sintered in a continuous sintering furnace, with any support being required during its residence in the sintering furnace to avoid collapse of the strip.

It has been suggested that the carrier could be in the form of an endless metal strip which passes through the sintering furnace.

German laid-open patent application 1583732 relates to a method for the continuous production of a metal strip, in which metal powder is compacted to form a raw strip and this raw strip is subsequently introduced into a sintering furnace, the strip being carried through a gas cushion during its passage through the sintering furnace.

German Offenlegungsschrift 2012794 discloses a similar process, however, wherein the raw strip is transported through the sintering furnace on a larger number of rolls. German Auslegeschrift 1483698 also relates to a similar arrangement, except that the metal strip is made of copper powder and transported through the sintering furnace by means of an endless belt. The German Auslegeschrift 1004457, the ffanz.Patentschrift no. 1.405.718, the British Patent No.1,067,208 and the United States Patent No. 2.834,674 relate to similar methods or Apparatuses as aforesaid, with the exception that the raw strip is made of a metal powder formed by the nip of a compaction roll assembly, but in all cases the raw strip is supported by device parts as it passes through the sintering furnace.

However, the previously known methods and apparatus for producing metal strips by sintering metal powder have not provided satisfactory results, u.zw. for the following reasons:

The sintering of the strip carried by a belt, due to the tensile stress applied to the compacted powder during sintering due to shrinkage of the strip by the strip as it passes through the sintering furnace, does not produce a strip having the desired mechanical properties. In particular, the friction inhibition applied by the belt to the strip during its shrinkage test results in ineffective sintering, which results in surface discontinuities of the strip during subsequent rolling.

In order to eliminate these disadvantages, according to the invention, the speed at which the strip leaves the sintering furnace is kept at a lower value than the value at which the strip enters the sintering furnace, the difference being that proportional to the expected linear shrinkage of the strip within the sintering furnace, whereby the tensile stress of the strip as it passes through the sintering furnace is approximately zero.

The metal powder can be fed into the gap between the oppositely rotating rolls of a compacting mill for the purpose of producing raw strips, wherein the raw strip can be carried before its entry into the sintering furnace by means of a levitation table. The raw strip is transported by means of two cooperating pickup rollers, which are arranged on exit from the sintering furnace, through the sintering furnace. The raw strip can be fed by means of pressure rollers in the sintering furnace, wherein the respective rotational speed of the pressure rollers and the pickup rollers are held in dependence on each other to keep the tension applied to the strip during its passage through the sintering furnace substantially to zero.

The term "substantially stress-free" as used in the specification is to be understood as an indication of a tensile stress applied to the compacted powder during its residence in the sintering furnace of a size which allows the sintered strip free shrinkage , The tensile stresses applied to a compacted ferritic or austenitic stainless steel powder are preferably less than 50 and 70 kN / m, respectively², For both powder materials, the applied tensile stress can advantageously be less than 15 kN / m² and preferably less than 10kN / m², based on the cross-sectional area, be. Pressure loads can advantageously be used for increasing the sintering time - 3 -

No.334718, when the application of these loads does not buckle the raw strip during its passage through the sintering furnace.

The gas cushion may be any gas or mixture of gases whose physical and chemical properties are compatible with the support system and the material to be treated. It can, for example, the gas cushion of argon; Nitrogen; or mixtures of argon and hydrogen; of nitrogen and hydrogen; or argon, nitrogen and hydrogen; or argon, hydrogen and methane. Preferably, the gas mixture consists of about 80% of the dense supporting gas (eg Argon and / or nitrogen).

After leaving the sintering furnace, the sintered strip may be subjected to cold rolling by 20% to reduce its thickness. The sintered and rolled strip may then be passed through a reheating furnace before being subjected to further forming rolling. The reheating furnace may be a sintering furnace, as essentially stated above, in which the strip is supported on a gas cushion as it moves through the furnace. Otherwise, the sintered strip may be recycled through the first-mentioned sintering furnace before performing another roll forming.

After each of the sintering, cold rolling and reheating stages, the strip can be wound up before being passed through subsequent stages. Otherwise, one or more of these stages may follow one another without an intermediate winding stage.

The apparatus for continuously producing a metal strip, means for compacting powder to form a green strip, means for transporting the compacted green strip through a sintering furnace, and means for introducing a gas into the sintering furnace to form a gas cushion therein for supporting the green strip whose passage through the sintering furnace comprises, is characterized in that control means are provided to keep the speed at which the strip leaves the sintering furnace at a lower value than the value of the speed at which the strip enters the sintering furnace, the difference between the two values ​​being the expected linear shrinkage of the sintering furnace Strip within the sintering furnace is proportional, whereby the tensile stress of the strip as it passes through the sintering furnace is approximately zero.

Further features and advantages of the invention will now be explained in more detail with reference to the drawings. Show it: 2 shows a cross section through a sintering furnace shown in FIG. 1, FIG. 3 shows a side view of the apparatus for cold rolling of the strip produced by means of the device according to FIG. 1, FIG 4 shows a side view of the device for reheating the rolled strip produced by means of the device according to FIG. 5 is a side view of a Sendzinierwalzwerkes for rolling the strip produced by means of the device according to Figure 4 and Figure 6 and 7 are partially sectional side views of other devices for the manufacture of metal strip according to the invention.

The apparatus shown in Fig.l and 2 comprises a hopper -1-, which contains a powder -P-. The powder may be made of ferrous material, eg of ferritic or austenitic stainless steel, of non-ferrous materials such as aluminum, metal-containing ore or metallic oxides. Immediately below the hopper -1- two press rolls -2- are arranged so that the powder emerging from the lower end of the hopper -1-, is drawn into the gap between the press rolls -2-. As shown, the press rollers -2- rotate in opposite directions of rotation, the whole arrangement of the press rollers -2- and the hopper -1- represents a press rolling mill, through which the raw strip -S- is generated. In the working direction after the press mill two cooperating pressure rollers -4-, a floating table -5-, a sintering furnace -6-, two cooperating pressure rollers -7- and a winding core -8- are provided. The wound strip is designated by the reference numeral -9-. As shown, the raw strip is fed from the press mill via the levitation table -5- and through the sintering furnace -6-, to be transported by this by means of the pressure rollers -4 and 7-. The respective rotational speed of the pressure rollers -4 and 7- are so interdependent that the tensile stress in the raw strip during its passage through the sintering furnace 6 is substantially zero; for a raw strip of austenitic stainless steel powder, the tensile stress would be less than 70 kN / m² Cross-sectional area and for ferritic powder to less than 50 kN / m² Cross-sectional area to be controlled.

Therefore, to compensate for the shrinkage of the strip during its passage through the sintering oven -6-, the respective rotational speed of the pressure rollers is synchronized by means of a control -10-, so that the rotational speed of the pressure rollers -7- by one of the expected linear reduction of the strip due the shrinkage during passage through the sintering furnace corresponding amount is smaller than that of the pressure rollers -4-. The expected shrinkage can be determined on the basis of the knowledge of the composition of the raw strip, the morphology of the metal powder and the conditions prevailing in the sintering furnace. For a strip made of stainless steel powder, a linear reduction of up to 5% may occur. As a rule, the linear reduction ranges between 1 and 2%. For one - 4 -

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Raw strip, which is made of materials such as metallic oxides, the linear reduction can be 30 to 40%. The setting of the controller -10- can be controlled automatically or manually in accordance with a quality control via a feedback circuit. In the other case, the tension in the raw strip can be removed in some places between the pressure rollers -4 and 7- and the tension thus determined can be introduced into the control -10- to effect a different control of the rotational speed of the pressure rollers. Preferably, the tensile stress is removed at some points within the sintering furnace.

The floating table -5- has a flat, horizontal plane and is arranged so that it

Gap between the pressure rollers - 4- and the sintering furnace -6- as far as possible ausflillt. The

Floating table -5- has a gas opening -11-- and many small (not shown) gas openings on its upper surface, by means of which the raw strip can be worn before it enters the sintering furnace. In another embodiment, the floating table -5- is replaced by a pneumatic chamber with inclined side walls similar to those used in the sintering furnace -6- and shown in Fig.2.

As can be seen more clearly from Fig. 2, the sintering furnace -6- has a refractory lining -12- and is provided with inlet seals -13- and outlet seals -14- which are arranged at the respective ends of the sintering furnace. Gas inlet openings -15- are arranged at intervals from each other along the bottom of the sintering furnace. In another case, the gas inlet openings -15- may be spaced apart along one or both sides of the sintering furnace -6-.

At least a portion of the gas in the sintering furnace can be withdrawn through a conduit 19 and the gas inlet openings 15 through a cooler 19A, a compressor 19B and a gas treatment chamber 19C in the contaminants, such as, for example, Oxygen, to be removed, to be fed back. An additional gas from a source -19D- of the required composition is added to the recirculated gas prior to its return to the sintering furnace. Before re-entering the sintering furnace, the re-supplied gas and the added gas are heated to a predetermined temperature.

Electric heating elements -16- are installed in the sintering furnace -6- along with one or more temperature controllers (not shown). Furthermore, a pair of lips 17- is provided, which runs horizontally along each vertical side wall of the sintering furnace. In another arrangement, the lips 17 may be inclined downwards at a smaller angle.

When leaving the sintering furnace -6- the cooled strip -9- passes through the pressure rollers --7- and is wound onto the winding core -8-.

The winding is then brought to a rolling station (Fig.3). As shown, the strip is passed through rollers -20- of a cold rolling mill -21- and rewound by means of a winding core -22- to form a coil from strip -23-.

In another embodiment, the strip is subjected to hot rolling prior to cooling and spooling. In this alternate embodiment, the hot rollers replace the pressure rollers -7-, with their rotational speed being controlled to maintain de-stress in the strip during its passage through the sintering furnace.

As can be seen from Figure 4, the strip -23 - after unwinding a

Reheat station fed, in which it is passed through a sintering furnace -24- and wound again to form a coil from the strip -25-.

The strip is conveyed to and from the sintering furnace -24- by pressure rollers -26- and take-up rollers -27-. The sintering furnace -24- can be identical to that in FIGS. 1 and 2. In another

In the embodiment, the strip - 25 - is returned to the sintering furnace -6- for reheating.

On the other hand, the sintering furnace may also comprise a continuously moving conveyor belt carrying the strip during transport through the sintering furnace.

Finally, the roll is conveyed from the strip -25- to a finishing mill (Fig. 5) in which the strip is rolled to a final gauge in a Z-mill (Z-mill) and rewound to form a finished roll.

In the operation of the apparatus shown, steel powder -P- is drawn from the hopper -1- into the gap between the press rolls - 2- and emerges therefrom as raw strip -S-. The raw strip is then from the pressure rollers - 4 over the horizontal surface of the floating table -5- by the

Entry seal -13- led into the sintering furnace -6- and leaves the sintering furnace at the exit seal -14-. The raw strip -S- is withdrawn from the sintering furnace by means of the pressure rollers -7- and wound up by means of the winding core -8. While the strip is in the sintering furnace -6-, it is carried by means of gas, which is applied by the inlet openings -15- under pressure on its underside. The contact of the edges of the strip with the lips, which are arranged along the side walls of the sintering furnace, is indicated by the gas between the strip edges and the inclined surface of the lips -17-, as indicated by the arrow -18- , can pass through, minimized or prevented. The gas leaves the sintering furnace through the line -19- will

No.334718 cooled, compressed, treated and reheated before passing through the gas inlet openings -15- in the

Sintering furnace is returned. Gas losses through the inlet seal -13- and the outlet seal -14- are counteracted by the addition of gas from the source -19D-.

The gas supplied through the gas inlet ports -15- may be a mixture of 20 vol.% Hydrogen and 80 vol.% Argon. In the other case, the mixture may consist of argon and a gas which reacts chemically with the strip; thus, to increase the nitrogen or carbon content of the metal powder from which the strip is made, the mixture may consist of argon and nitrogen or argon and a hydrocarbon gas such as methane. Thus, to increase the nitrogen content of an austenitic stainless steel strip by 0.2%, the gas composition may include 25% nitrogen, 55% argon and 20% hydrogen. For a strip made of stainless steel powder, the temperature in the sintering furnace is maintained by the heating elements -16- at approximately 1350 ° C, so that the raw strip -S- is sintered at the correct temperature. While the strip is in the sintering furnace -6-, the tension applied to the strip is kept substantially zero due to the gas cushion supporting it and the aforementioned synchronized, dependent rotational speed of the pressure rollers -4 and 7-.

The sintered strip is pulled out of the sintering furnace -6- by means of the pressure rollers -7- and the winding core -8-. The resulting coiled strip -9- is then brought to the cold rolling mill -21-, in which the strip is unwound and passed through the rollers -20- and then rewound by means of the winding core -22-. The rollers -22- cause a 20% reduction in strip thickness.

The wound strip -23- is then passed through the Wiedererhitzungsstrecke shown in Figure 4; the unwound strip is guided through the sintering furnace -24- by means of the pressure rollers -26 and 27- and then rewound to obtain the wound strip -25-. The wound strip -25- is fed into the Z-mill, where it is rolled to a final thickness and wound up. The reduction in thickness achieved in the Z mill is usually 35%, but can be considerably more than determined by final strength and properties.

In an embodiment not shown, the sintered strips leaving the pressure rollers 7 are introduced directly into the cold rolling mill -21- without an intermediate winding station.

Additionally or alternatively, the rolled strip leaving the cold rolling mill may be passed directly through the sintering furnace without a winding station becoming intermediate between the rolling line (FIG. 3) and the reheating section (FIG. 4). For heating the sintering furnace -6 and 24- no electrical device must be provided; The heating device may, for example, also be a high-frequency induction device or an electron beam device.

Fig. 6 shows another way in which the tensile stress in the raw strip -S- during its transport on a gas cushion through the sintering furnace -6- can be kept substantially at zero. In this other arrangement, in which the same reference numerals have been used as in Fig.l, a driven friction drum -30- between the press rolls -2- and the floating table -5- is arranged. The motor driven friction drum has an outer drum shell of friction material, which is conventionally made of cellular elastomer such as foamed polyurethane. Such a material providing a frictional drive between its surface and the overflowing raw strip is also repellent to powder adhesion. Loose powder, which during the

Passage of the strip ns from the surface of the friction material - 31 - would fall into the open pores of the friction material and be removed therefrom by gravity as that portion of the friction drum -30- comes out of contact with the raw strip -S - moved away.

At start-up, the raw strip -S- emerging from the press rolls is passed through the sintering furnace -6- over a portion of the outer roll surface of the friction material -31- on the drum, over the upper surface of the levitation table -5-. and two mutually rotating pick-up rollers -32- led to the winding core -8-.

As in the arrangement according to FIG. 1, the raw strip is carried by means of a gas cushion while it is in the sintering furnace -6-, the gas being fed through the inlet openings -15-. As mentioned above, it is important that the tensile stress generated in the raw strip -S- be kept substantially zero. In the arrangement shown in Fig. 6, the raw strip is suspended in a small loop -L- between the press rolls and the friction drum -30-, but the rotational speed of the friction drum -30- is such with the rotational speed of the press rolls -2-, the take-up rolls. 32- and the winding core -8- held in dependence that the tensile stress occurring in the raw strip is substantially equal to zero. The tensile stress in the raw strip remains substantially zero, starting from the friction drum -30-, while passing through the floating table -5- and the sintering furnace -6-.

- 6 -

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In operation, the friction drum -30- is driven at a peripheral speed which is slightly greater than the speed at which the raw strip -S- exits from the press rollers -2-. The voltage in the strip as it enters the sintering furnace -6- can be regulated to zero by regulating the height of the loop -L-. This control is achieved by controlling the peripheral speed of the take-up rollers -32- and the winding core -8- at the exit end of the sintering furnace -6-. Such speed settings can be made automatically by responding to a suitable sensor. For example, the sensor may indicate the stress condition in the strip as it passes the float table.

Figure 7 shows a further device to keep the tension in the raw strip -S- during its transport through the sintering furnace -6- substantially to zero.

In this other arrangement, a curved, downwardly sloping floating table -35- is arranged between the press rolls -2- and the inlet opening -41- of the sintering furnace -6-. Gas is passed through a line -36- to the floating table -35-.

The sintering furnace -6- is inclined to the horizontal by a small angle to allow passage of the strip through the sintering furnace under gravity.

The angle of inclination to the horizontal is such that the frictional resistance of the strip passing through the sintering furnace 6 is canceled by the force of gravity on the strip. The angle may be on the order of 0.5 to 5 ° and is achieved by placing the inlet opening -41- of the sintering furnace higher than the outlet opening 42-. A sensor -39- determines the distance between the floating table -35- and the raw strip -S-.

The raw strip is moved through the sintering furnace by means of two oppositely rotating pickup rollers -37-, wherein the rotational speed of these pickup rollers is controlled by a controller to the voltage in the raw strip at signals from the sensor -39-, the voltage in the raw strip -S- indicates to essentially zero. In this way, the tension in the raw strip can be maintained at a desired value.

Although the invention has been described with reference to the manufacture of metal strips from a green strip made by passing metallic powder through a press mill, it will be understood that other methods of making the green strip from a powder raw material may be employed. Such another method comprises depositing a slurry coating containing a suspension of powdered material in a binder composition, drying the slurry on the carrier surface on the carrier surface to form a dried, self-supporting film, removing the dried film from the carrier surface, and rolling the dried film to densify and form a green strip.