Production of metal strip

The subject of the invention is a method for the continuous production of a metal strip and a device for the continuous production of a metal strip, in particular the production of a metal strip by crushing the metal powder between rolls (corrugation).

There are known methods of continuously producing metal strip and other shapes by squeezing the powder between rolls. In these methods, cold crushing alone is not sufficient to produce a strip with a density and strength similar to that of ingot-rolled strip. * It is necessary to sinter the crushed powder; which is heated to a temperature at which the powder binds by melting its grains or by solid-state diffusion at high temperature. ,

Sintering may be supplemented with further rolling and heat treatment in order to obtain a strip with suitable mechanical properties and final surface condition. In this way, a strip with sufficient density and mechanical properties comparable to those of the strip cast from the ingot is obtained.

Preferably, the strip is sintered in a continuous furnace provided with a stop surface to prevent strip breakage. Hitherto, a support surface has been proposed in the form of an endless metal strip that travels through a furnace.

Attempts to produce a tape in this way met with a number of serious difficulties. Sintering the tape supported on the belt does not work. as a result of the belt having the desired mechanical properties, since tensile stresses appear in the belt due to the action of the belt limiting the free contraction of the belt as it passes through the furnace. The 'frictional resistance which arises as a result of contact between the belt and belt surfaces', with simultaneous belt shrinkage, leads to ineffective sintering manifested by cracks in the belt surface during subsequent rolling.

The object of the invention is 'to provide a method for the continuous manufacture of a metal strip comprising crushing the powder to form a green strip, feeding the raw strip to a roasting furnace and holding' while the strip is transported through the furnace of a support gas cushion, the transport of the strip being controlled so that the tensile stress in the belt as it passes through the furnace is substantially zero.

A further object of the invention is to provide a device for the production of a metal strip, which has means for compacting the powder into a raw strip shape, a means for transporting the formed raw strip through the roasting furnace, means for supplying gas to the roasting furnace to produce a gas cushion on which the green rests. the tape passing through the furnace, and means controlling the means (Transporting the tape in this way,

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394 that the tensile stress in the belt as it passes through the furnace interior is substantially equal to zero.

This object is achieved in that the green strip is formed by passing metal powder between two counter-rotating rolls of a compacting mill; before entering the roasting kiln, the raw belt rests on the air table. The raw belt is guided through the roasting kiln by means of a pair of intermeshing rollers positioned at the exit of the kiln. Raw tape is introduced. to the roasting furnace by means of two interacting nip rolls, the rotation speed ratio of the nip rolls to the take-up rolls being kept relative to such a value that the tensile stress in the belt as it passes through the furnace is substantially equal to zero.

The term "substantially equal to zero" as used herein refers to such a tensile stress in the crushed powder, sintered in the oven, which allows the sintered strip to shrink freely. The tensile stresses in the crushed powder of ferritic and austenitic stainless steel are preferably less than 50 and respectively 70 kilonewtons / m². For both powdered materials, a suitable tension is less than 15 kilonewton / m2, preferably less than 10 kilonewton / m2. A compressive stress of a size that does not exaggerate the green belt passing through the furnace may also be applied.

The cushion gas may be any gas or mixture whose physical and chemical properties are compatible with the gas cushion system and the material being processed. For example, a gas cushion may contain argon, nitrogen, or a mixture of argon, nitrogen, and hydrogen, or argon, hydrogen, and methane. Preferably the mixture contains about 80% dense gas, i.e. argon and / or nitrogen. .

After exiting the roasting furnace, the sintered strip is subjected to a cold rolling operation with a thickness reduction of 20%. The sintered, rolled strip is passed through a heating furnace prior to further rolling to its final thickness. The reheating oven may be identical to the aforementioned roaster in which the conveying belt rests on a gas cushion. Alternatively, the sintered and rolled strip may be re-passed through the aforementioned roaster before further rolling to final thickness. After each of the sintering, cold rolling, and annealing operations, the strip can be rolled up before being delivered to the next. positions. Alternatively, the stations may be arranged one after the other without coiling stations separating them.

The subject of the invention is shown in the drawing in which exemplary embodiments are shown, in which Fig. 2 shows a device for producing a metal strip in a side view and a partial section. Fig. 2 - a sintering furnace according to Fig. 1 in cross-section, Fig. cold rolling of the strip produced according to Fig. 1 in a side view; Fig. 4 shows a device for reheating the rolled strip produced with the device of Fig. 3 in the side view; fig. 5 - a Sędzimir mill for rolling a strip produced with the device according to fig. 4 in a side view, and figs. 6 and 7 an exemplary embodiment of a device for producing a metal strip according to the invention in a side view and a partial section.

The device shown in Figures 1 and 2 comprises a hopper 1 with a powder "P". The powder may be made of an iron-containing material, preferably a ferritic or austenitic stainless steel, a non-iron-containing material, e.g. aluminum, or a metal containing a metallic oxide or ore. Immediately below the hopper 1 there is a pair of forming shafts 2, 2 so that the powder is poured out. the funnel goes straight into the grip of the forming rollers 2,

2. As shown in the yyisunku wacee formers 2, 2 can only rotate in opposite directions, and the set of forming rolls 2, 2 and hopper 1 form a / rolling mill that compacts the powder and produces a raw and "S" band therefrom.

Below the compacting mill, they are placed successively; a pair of 'interacting feed rollers 4, 4, an air table 5, a roasting furnace 6, a pair of interacting pick-up grippers 7, 7 and a belt coiler 8.

The folded strip is indicated by reference numeral 9. The raw strip "S" is passed over the air table 5 through the roaster 6 by means of the nip pairs of rolls 4, 4 and 7, 7. The ratio of the rotational speeds of the insertion rolls 4, 4 and the nip take-off rolls 7, j ^ t is such that the tensile stress in the green belt passing through the roasting furnace 6 is substantially close to zero; for raw strip made of austenitic stainless steel powder, the tensile stress is maintained at a level not exceeding 70 kN / m2 in cross-section, and for ferritic powder at a level not exceeding 50 kN / m2 in cross-section. Thus, to compensate for the shrinkage of the strip in the roasting furnace 6, the speed ratio of the rotational nip rolls to one another is adjusted by the controller 10 such that the rotational speed of the take-up nip rolls 7, 7 is less than the speed of the nip feed rolls 4, 4 by a quantity proportional to anticipated linear reduction in the length of the strip as a result of the shrinkage as it passes through the roaster.

The predicted shrinkage can be determined from the knowledge of the raw strip composition, the grain structure of the metal powder and the conditions in the roaster. For a strip made of stainless steel powder, the linear shrinkage can be up to 5%. Typically, the linear shrinkage is in the order of 1 to 2%. For a raw strip made of materials such as metallic oxides, the linear shrinkage can be as high as 30 to 40%. The settings of the Controller 10 may be set manually or automatically by the feedback branch from the product quality control unit.

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Alternatively, the tensile stress can be measured at - the location between the nip insertion rolls 4,. 4 and the nip rollers 7, 7, and the received signal can be input into the controller 10 for regulating the speed difference of the rotating nip rolls 4, 4 and 7, 7. Preferably, the belt tensile stress sensor is placed inside the oven.

Air table 5 - has a flat horizontal surface and is positioned to occupy as little free space as possible between the nip insertion rolls 4, 4 and the roaster 6. Table 5 has a gas inlet 11, and a plurality of small outlets (not shown). on the gray top surface that supports the raw belt before it enters the kiln.

In an alternative embodiment, the air table 5 is replaced with an air chamber with sloping side walls similar to those used in the roaster 6 shown. in Fig. 2. t

In Figure 2, the roaster 6 has a refractory lining 12 'and is provided with an entrance sealing member 13 and an exit sealing member 14 at each end of the oven. Gas inlet ports 15 are located along the bottom of the furnace. Alternatively, the gas inlet ports 15 may be positioned on both wounds of the roaster 6.

At least a portion of the gas contained in the roaster 6 may be removed through conduit 19 and returned to the inlet ports, via cooler 19A, compressor 19B and chamber 19C. _in contaminants such as oxygen are removed. An additional portion of gas with the required composition is added to the recirculating gas before it is reintroduced into the furnace. The gas from the recirculation system and additional gas are heated to the specified temperature before it is recirculated to the furnace.

Inside the roasting furnace 6, electrical elements 16 are located together with one ltfb and several temperature controls. A pair of air-runners 17 run horizontally along each of the lateral vertical walls of the kiln. Alternatively, the air-runners 17 may be inclined downwardly by a slight angle.

After exiting the roasting furnace 6, the strip is cooled, then it passes through the receiving nip rollers 7, 7 and is wound on a winder 8 to form a sheet 9.

The strip coil 9 is transferred to a roll station shown in FIG. 3. As shown, the strip passes through the rolls 20 of mills 21 when cold and is rewound with a coiler 22 to form a strip coil 23.

Alternatively, the strip is heated prior to cooling and winding. In this solution, the nip take-off rollers 7, 7 are replaced by hot rollers rotating at a speed corresponding to the tensile stress of the belt passing through the interior of the furnace equal to substantially zero.

In Fig. 4, the coil of the strip 23 after coiling is transferred to the (intermediate) annealing station, where the strip passes through the furnace 24 and is re-wound into a coil 25. The feeding into the furnace and the removal of the strip from the furnace 24 is accomplished by using nip rolls 26. and - receiving rolls 27. The furnace 24 may be the same as shown in Figs. 1 and 2. Alternatively, the coil 25 is annealed in the roaster 6. Alternatively, the furnace may be_x include a conveyor belt on which a sandwich belt is processed during transport in the oven 24.

Fig. 5 shows a strip of strip 25 which is brought to the end of the rolling strip, on which the strip is rolled to its final thickness in a Sędzimir mill ("Z" roll) 28 and coiled into a final roll 29.

During the operation of the device shown in the drawing, the steel powder "P" is pulled from the hopper 1 of the feeder into the nip between the forming rolls 2, 2, where the raw belt "S" is formed. The strip is introduced into the roasting furnace 6 by means of nip rollers 4, 4 above the horizontal surface of the air table 5 through the entrance seal 13. The tape * leaves the roaster 6 through, seal 14. *

Inside the roasting furnace 6, the belt rests on a pillow produced by the supply of gas under pressure through the Tdot port 15. The contact between the edges of the belt and the air sills 17 located along the side walls of the oven is minimal or completely disappears due to the gas flow between the edges of the strip and the inclined air sills 17, as shown in the drawing by arrow 18. The gas leaves the furnace via line 19 and is then cooled, compressed, purged, and reheated before being re-introduced into the furnace through the inlet ports 15. The gas losses resulting from the flow through the inlet 13 and outlet 14 seals are replenished by adding additional gas from source 19D.

The gas supplied through the inlet ports 15 may be a mixture containing 20% ​​hydrogen and 80% argon by volume. Alternatively, the mixture may contain argon and a gas chemically reacting with the strip and thereby increasing the nitrogen or carbon content in the metal powder of which the strip is made. The mixture may contain argon and nitrogen or argon and a hydrocarbon, preferably methane. Thus, to increase the nitrogen content of the austenitic stainless steel strip by 0.2% - the mixture should contain 25% nitrogen, 56% argon and 20% hydrogen.

In the case of a strip made of stainless steel powder, in order to ensure the proper sintering temperature of the strip "S", the heating elements 16 maintain - in the furnace - the temperature of approximately 1350 ° C. Thanks to the base of the tape on the air cushion and the mentioned. above the synchronization of the rotational speeds of the nip rollers 4, 4 and 7, 7. The tension in the belt during the time it is in the oven is substantially zero.

The sintered strip is pulled out of the roasting furnace 6 by means of receiving rollers 7, 7 and a strip winder 8. The resulting strip 9 is delivered to rolling mill 20 when cold, where the strip is unwound, passes through rollers 20, 20 and is rewound by the strip winder 22. The rollers 20, 20 reduce the thickness of the strip by 20%.

The web 23 then passes through the annealing furnace shown in Fig. 4. The strip is unwound, passed through the furnace 24 guided by nip rollers 20, 26 and 27, 27 and then coiled into a web 25.

Finally, coil 25 is fed to rolling mill 28 _: "Z" type, where the strip is rolled - - to its final thickness and coiled again. The strip thickness reduction in a Z-type rolling mill is in the order of 35%, but may be substantially greater according to the desired final strip thickness and properties. .

In an exemplary embodiment which is not shown in the drawing, the "baked strip leaving the nip rollers 7, 7" is fed directly to the rolling mill 21, bypassing the - indirect coiling operation. In addition, or only alternatively, the re-rolled strip exiting the rolling mill 21 is fed directly into the furnace 24 bypassing the coiling steps between the rolling line shown in Fig. 3 and the annealing furnace <Fig. 4). :

The heating means used to heat the furnaces 6 and 24 need not be an electrical device, but e.g. a high frequency induction device, such as an electron beam device. :

Fig. 6 shows an alternative method of obtaining substantially-zero tensile stress in the green belt "S" while it is transported on a gas cushion through the roasting furnace β. In Fig. 6, the elements are similar to those of the elements_;in Figure 1 are provided with the same reference numerals. In this solution, a driven friction drum 30 is arranged between the forming rolls 2, 2 and the air table 5.

The friction drum 36 provided with its own drive motor has a lining on its outer periphery made of a friction material 31, preferably a cellular matrix, such as, for example, foamed polyurethane. The material - this provides a frictional drive between its surface and the raw tape passing over it, and a structure that prevents the powder from being trapped in it. The powder 'hydrated from the tape and attached to the surface of the cellular material 31' falls into .. the interior of the open pores from which it spills out under the action of gravity in the area where the bubble surface is not in contact with the tape.

Raw "S" tape during operation. starting from the forming rolls, it passes along the outer periphery of the covering material 31, the drum 30 along a part of the circumference of this drum, then it passes over the surface of the air table 5, then through the roasting furnace 6 and between ... a pair of counter-rotating receiving rolls 32, 32 hits the coiler 8. As in Fig. 1 the raw belt in the roasting kiln 6 rests on a gas cushion produced by the gas inlet ports 15.

As mentioned above, it is important that the belt's tensile strength and tensile stress are kept substantially at zero. In the example shown in Fig. 6, the green strip forms a slight overhang "L" between the forming rolls and the friction drum 30, and the rotational speed of the friction drum 30 remains in relation to the rotational speeds of the forming rolls 2, 2 of the rotating take-up rolls 32, 32. and a coiler 8 that the tensile stress in the green belt is substantially equal to zero. This stress remains at a substantially zero level also below the friction drum 30 as it passes through the air table 5 and through the roasting furnace 6.

During operation, the friction drum 30 is driven at such a speed that its surface speed is slightly higher than the speed at which the green belt "S" exits the forming rolls 2, 2. The stress in the belt passing through the roasting furnace - 6 may be adjustable to a level (desired zero by changing the height of the overhang "L". This change is achieved by adjusting the rotational speeds of the receiving rollers 32, 32 and the coiler 8 at the exit of the roasting furnace 6. The regulation of said rotational speeds may be automatic based on the signal of a suitable sensor; the sensor may e.g. indicate the value of the tension in the belt where it passes over the air table 5.

Fig. 7 shows a given embodiment of the device, in which the principle of maintaining at the level of bHilkm zero stress in phtaśmde "S" during its passage through the calcining furnace 6, was carried out differently. '

In this solution, a downwardly inclined, curved Air Table 35 is arranged between the forming rolls 2, 2 and the inlet port 41 of the roasting furnace 6.

The working gas is supplied to the air table 35 via the conduit 36. The roasting furnace 6 is inclined with respect to the horizontal by a small angle to allow the belt to be displaced under the action of gravity. The angle of inclination with respect to the horizontal is such that the frictional resistance of the strip passing through the inside of the roasting furnace 6 is balanced by the force of gravity of the strip. This angle of 0.5 ° to 5 ° is obtained by cleaning the inlet port 41 of the furnace higher than the outlet port. The sensor 39 measures the distance - between the air table 35 and the "S" band.

The belt passes through the furnace pulled by a pair of counter-rotating (receiving rollers 37, the speed of rotation of which is controlled by a regulator 40 operating on the basis of a sensor signal 39 indicating the tension in the i-S-belt. This adjustment is intended to maintain a desired substantially zero tension level). in the tape "S". *

While the invention has been described with reference to a method of producing a metal green strip by passing the metal powder through forming rolls, it should be understood that other methods of producing a raw strip of a powdered starting material may also be used. One such method involves the operation of applying to a horizontal surface supporting a layer - a slurry containing the powder material dispersed in the binding mixture, an operation of drying the slurry to form a dry self-supporting layer, an operation of removing the fried layer from the substrate surface, and rolling the dry layer to thicken it. and forming a raw strip.