GB2093482A - Continuous Production of Sintered Metal Strip - Google Patents

Abstract

Sintered strip is made by roll compacting metal powder sintering the green strip supported on a gas cushion and subjecting the surface of the sintered strip to hot working prior to cooling to increase the density and to improve the mechanical properties of the strip. Hot working is effected by impacting shot material on to one or both surfaces of the strip as the strip passes between sintering and cooling chambers of a furnace.

Description

SPECIFICATION Continuous Production of Metal Strip This invention relates to the continuous production of metal strip and more especially to the production of steel strip by roll compacting metal powder.

Processes have been proposed for the continuous production of metal strip by compaction alone is in sufficient to produce strip of a density and strength approaching that of strip density and strength appcoaching that of strip rolled from an ingot. The compacted powder needs to be sintered, that is, heated to a temperature at which the powder particles tend to bond together by liquid phase diffusion or high temperature solid state diffusion and consolidated further by hot or cold rolling to achieve full density.

Our UK Patent Specification 1466364 discloses and claims a method and apparatus for continuously producing metal strip in which a green strip produced by roll compacting metal powder is supported by a gaseous cushion, is transported through a sinter furnace, the strip being transported in such a way as to permit the strip to shrink linearly as it passes through the fumace. The specification further discloses several process routes by which the sintered strip may be reduced to the final gauge required for the product.

The present invention sets out to provide an improved method of producing strip material by roll compacting metal powder, in which the surface of the sintered strip is subjected to hot working prior to cooling to increase the density and hence improve the mechanical properties of the strip.

According to the present invention in one aspect, there is provided a method for the continuous production of metal strip which includes compacting powder to form a green strip, feeding the green strip to a sinter furnace and supporting the strip by a gaseous cushion as it is transported therethrough, impacting shot material onto one or both surfaces of the strip to hot work the same and subsequently cooling the sintered strip. Preferably, shot material is impacted onto each surface of the sintered strip as it passes between heating and stress relieving or cooling zones of the sintered furnace or as it travels through said cooling zone.

According to the present invention in another aspect, there is provided apparatus for the continuous production of metal strip which comprises means for compacting powder to form a green strip, means for transporting the compacted green strip through a sinter furnace, means for feeding gas to the sinter furnace to produce a gaseous cushion to support the green strip as it passes through the heating zone of the sinter furnace, means for impacting shot onto one or both surfaces of the sintered strip to hot work the same and means for cooling the hot worked strip.

The invention will now be described by way of example with reference to the accompanying diagrammatic drawing in which the sole figure is a side elevational view partially in section of apparatus for producing metal strip in accordance with the invention.

The apparatus illustrated includes a hopper 1 which contains powder "P". The powder may be manufactured from a ferrous material, for example ferritic or austenitic stainless steels, a non-ferrous material, such as aluminium, a metal bearing ore or a metallic oxide. Immediately below the hopper 1 a pair of compacting rolls 2 are arranged so that the powder which leaves the lower open end of the hopper 1 is drawn into the nip between the rolls 2. As illustrated, the rolls 2 are constrained to rotate in opposite directions and the whole assembly of rolls 2 and hopper 1 comprises a compaction mill from which green strip "S" is produced.

Downstream of the compaction mill are provided, in order, a pair of co-operating entry rolls 4-4, a flotation table 5, a sinter surface 6 including a heating zone 7, a shot impaction unit 14 and a cooling zone 8, a pair of co-operating exit pinch rolls 9, and a strip coiler 11. The entry rolls 4 effectively isolate the green strip "S" entering the furnace 6 from tensile stresses imposed in the strip upstream of the furnace and the exit rolls 9 effectively isolate the strip from tensile forces imposed in the strip downstream of the rolls 9. The coiled strip is indicated by the reference number 10. As illustrated, the green strip S from the compaction mill is fed over the flotation table 5 and through the furnace 6 to be propelled therethrough by the pairs of entry and exit rolls 4 and 9 respectively.

The respective speeds of rotation of the entry rolls 4 and exit rolls 9 are so interrelated that the tensile stress obtaining in the green strip as it passes through the sinter furnace 6 is substantially zero; that is to say, the tensile stress applied to the strip of a value which permits the sintering strip to shrink linearly. For a green strip produced from an austenitic stainless steel powder, the tensile stress would be controlled to a value of less than 70 Kilo Newtons per square metre of cross-section and for a ferritic powder, to a value of less than 50 Kilo Newtons per square metre of cross-section.Thus, in order to accommodate shrinkage of the strip as it passes through the sinter furnace 6, and possibly some elongation during shot impaction, the respective speeds of rotation of the en'try and exit rolls are synchronized by a controller 12 so that the rotational speed of the rolls 9 is related to that of the rolls 4 to accommodate any linear changes in the strip as it passes through the furnace.

The expected shrinkage can be determined from a knowledge of the composition of the green strip, morphology of the metal powder and the conditions obtaining in the sinter furnace. For a strip produced from a stainless steel powder, a linear reduction of up to 5% may occur. Normally this linear reduction would be of the order of 1 to 2%. For a green strip produced from materials such as metallic oxides the linear reduction may be as great as 30 to 40%. Any linear change in the strip during shot impaction can be determined from a knowledge of the change in density achieved by such impaction. The setting of the controller 12 may be automatically or manually control led in accordance with a quality control via feed-back circuitry.Alternatively, the tensile stress within the strip may be sensed at some location intermediate the rolls 4 and 9 and the stress so determined fed to the controller 12 to effect differential control of the rotational speeds of the rolls. Preferably the tensile stress would be sensed at some location within the furnace.

The flotation table 5 has a flat horizontal surface and is positioned so as to occupy as much as possible of the intervening space between the entry rolls 4 and the fumace 6. The table 5 has a gas inlet and many small gas outlets (not shown) on its upper surface so as to support the green strip thereon prior to its entry into the furnace.

Positioned between the heating and cooling zones 7,8 of the furnace is the sealed shot impaction unit 14 which includes a pair of shot impellers driven by motors 16 and supplied with shot from feeders 1 7. The impellers are positioned one above and one below the strip, used shot being collected in a collection unit. The collected shot may subsequently be cooled, cleaned and recycled to the feeders 17.

On leaving the cooling zone 8 the strip passes through the exit rolls 9 and is coiled by the strip coiler 1 The coiled strip may subsequently be further reduced within cold or hot rolling mills.

In an alternative unillustrated embodiment, the shot impaction unit is sited between a heating zone and a stress relieving zone of a furnace, the stress-relieved strip then passing to and through a cooling zone. Alternatively, the shot impaction unit may be sited within the cooling zone of the furnace. The undersurface of the strip may be supported by a moving support as it passes through the impaction unit, only the upper strip surface then being shot impacted.

In operation of the apparatus illustrated, steel powder "P" from the hopper 1 is drawn into the nip between compaction rolls 2 and emerges as green strip "S". The strip is then guided by the entry rolls 4 over the horizontal surface of the flotation table 5 into the heating zone of the furnace 6 via entry seal 18 and leaves the cooling zone of the furnace by exit seal 19. The strip "S" is drawn from the furnace by the exit rolls 9 and coiled by means of the strip coiler 11.

As the strip passes between the heating and cooling chambers its surfaces are subjected to hot working by the shot from the impaction unit.

The hot working increases the density of the sintered strip.

Whilst in the furnace 6, the strip is supported by means of gas supplied under pressure through gas inlet ports 21. Contact between the edges of the strip and strip support surfaces located along the side walls of the furnace is minimised or prevented by gas which is permitted to flow between the strip edges and the support surfaces.

The gas leaves the furnace through a conduit and is cooled, compressed, treated and reheated before being returned to the furnace through the entry ports 15. Gas losses through the entry and exit seals 18, 19 are compensated for by addition of gas from a suitable source.

The gas supplied through the inlet ports 1 5 may comprise a mixture consisting of 20% by volume hydrogen and 80% by volume argon.

Alternatively, the mixture may comprise a mixture of argon and a gas which reacts chemically with the strip. Thus, in order to increase the nitrogen or carbon content of the metal powder from which the strip is made, the mixture may consist respectively, of argon and nitrogen or argon and a hydrocarbon gas such as methane.

For a strip produced from a stainless steel powder the furnace temperature is maintained at approximately 1 3500C so that the strip "S" is sintered at the correct temperature. Whilst in the furnace 6, the tensile stress applied to the strip is maintained substantially zero due to the gas cushion on which it is supported and to the aforementioned synchronised interrelated rotational speeds of the entry rolls 4 and the exit rolls 9.

In an alternative unillustrated arrangement a curved downwardly inclined flotation table is positioned between the compaction mill and the inlet to the furnace 6. Furthermore, the furnace hearth may be tilted through a small angle to the horizontal to enable the strip to flow through the furnace under gravitational force. In this arrangement a sensor is provided to determine the height of the strip above the curved flotation table and this determined value is used to control the rotational speed of the rolls 9 in a sense to maintain the tension in the strip substantially zero.

Although the invention has been described with reference to the production of metallic strip from a green strip produced by passing metallic dry powder through a compaction mill it is to be understood that other methods of producing the green strip from a powder start material could be employed. One such alternative method includes the steps of depositing on a support surface a coating of a slurry comprising the suspension of powdered material in a combined composition.

The deposited slurry coating is then dried on the support surface to form a self-supporting film which is subsequently removed and rolled to form a green strip.

Claims (Filed 12 Feb 1982) 1. A method for the continuous production of metal strip which includes compacting powder to form a green strip, feeding the green strip to a sinter furnace and supporting the strip by a gaseous cushion as it is transported therethrough, impacting shot material onto one or both surfaces of the strip to hot work the same and subsequently cooling the sintered strip.

2. A method as claimed in Claim 1 wherein shot

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. such as metallic oxides the linear reduction may be as great as 30 to 40%. Any linear change in the strip during shot impaction can be determined from a knowledge of the change in density achieved by such impaction. The setting of the controller 12 may be automatically or manually control led in accordance with a quality control via feed-back circuitry. Alternatively, the tensile stress within the strip may be sensed at some location intermediate the rolls 4 and 9 and the stress so determined fed to the controller 12 to effect differential control of the rotational speeds of the rolls. Preferably the tensile stress would be sensed at some location within the furnace. The flotation table 5 has a flat horizontal surface and is positioned so as to occupy as much as possible of the intervening space between the entry rolls 4 and the fumace 6. The table 5 has a gas inlet and many small gas outlets (not shown) on its upper surface so as to support the green strip thereon prior to its entry into the furnace. Positioned between the heating and cooling zones 7,8 of the furnace is the sealed shot impaction unit 14 which includes a pair of shot impellers driven by motors 16 and supplied with shot from feeders 1 7. The impellers are positioned one above and one below the strip, used shot being collected in a collection unit. The collected shot may subsequently be cooled, cleaned and recycled to the feeders 17. On leaving the cooling zone 8 the strip passes through the exit rolls 9 and is coiled by the strip coiler 1 The coiled strip may subsequently be further reduced within cold or hot rolling mills. In an alternative unillustrated embodiment, the shot impaction unit is sited between a heating zone and a stress relieving zone of a furnace, the stress-relieved strip then passing to and through a cooling zone. Alternatively, the shot impaction unit may be sited within the cooling zone of the furnace. The undersurface of the strip may be supported by a moving support as it passes through the impaction unit, only the upper strip surface then being shot impacted. In operation of the apparatus illustrated, steel powder "P" from the hopper 1 is drawn into the nip between compaction rolls 2 and emerges as green strip "S". The strip is then guided by the entry rolls 4 over the horizontal surface of the flotation table 5 into the heating zone of the furnace 6 via entry seal 18 and leaves the cooling zone of the furnace by exit seal 19. The strip "S" is drawn from the furnace by the exit rolls 9 and coiled by means of the strip coiler 11. As the strip passes between the heating and cooling chambers its surfaces are subjected to hot working by the shot from the impaction unit. The hot working increases the density of the sintered strip. Whilst in the furnace 6, the strip is supported by means of gas supplied under pressure through gas inlet ports 21. Contact between the edges of the strip and strip support surfaces located along the side walls of the furnace is minimised or prevented by gas which is permitted to flow between the strip edges and the support surfaces. The gas leaves the furnace through a conduit and is cooled, compressed, treated and reheated before being returned to the furnace through the entry ports 15. Gas losses through the entry and exit seals 18, 19 are compensated for by addition of gas from a suitable source. The gas supplied through the inlet ports 1 5 may comprise a mixture consisting of 20% by volume hydrogen and 80% by volume argon. Alternatively, the mixture may comprise a mixture of argon and a gas which reacts chemically with the strip. Thus, in order to increase the nitrogen or carbon content of the metal powder from which the strip is made, the mixture may consist respectively, of argon and nitrogen or argon and a hydrocarbon gas such as methane. For a strip produced from a stainless steel powder the furnace temperature is maintained at approximately 1 3500C so that the strip "S" is sintered at the correct temperature. Whilst in the furnace 6, the tensile stress applied to the strip is maintained substantially zero due to the gas cushion on which it is supported and to the aforementioned synchronised interrelated rotational speeds of the entry rolls 4 and the exit rolls 9. In an alternative unillustrated arrangement a curved downwardly inclined flotation table is positioned between the compaction mill and the inlet to the furnace 6. Furthermore, the furnace hearth may be tilted through a small angle to the horizontal to enable the strip to flow through the furnace under gravitational force. In this arrangement a sensor is provided to determine the height of the strip above the curved flotation table and this determined value is used to control the rotational speed of the rolls 9 in a sense to maintain the tension in the strip substantially zero. Although the invention has been described with reference to the production of metallic strip from a green strip produced by passing metallic dry powder through a compaction mill it is to be understood that other methods of producing the green strip from a powder start material could be employed. One such alternative method includes the steps of depositing on a support surface a coating of a slurry comprising the suspension of powdered material in a combined composition. The deposited slurry coating is then dried on the support surface to form a self-supporting film which is subsequently removed and rolled to form a green strip. Claims (Filed 12 Feb 1982)

1. A method for the continuous production of metal strip which includes compacting powder to form a green strip, feeding the green strip to a sinter furnace and supporting the strip by a gaseous cushion as it is transported therethrough, impacting shot material onto one or both surfaces of the strip to hot work the same and subsequently cooling the sintered strip.

2. A method as claimed in Claim 1 wherein shot

material is impacted onto each surface of the sintered strip as it passes between heating and stress relieving or cooling zones of the sinter furnace.

3. A method as claimed in Claim 1 wherein shot material is impacted on to each surface of the sintered strip as it travels through a cooling zone of the sinter furnace.

4. Apparatus for the continuous production of metal strip which comprises means for compacting powder to form a green strip, means for transporting the compacted green strip through a sinter furnace, means for feeding gas to the sinter furnace to produce a gaseous cushion to support the green strip as it passes through the heating zone of the sinter furnace, means for impacting shot onto one or both surfaces of the sintered strip to hot work the same and means for cooling the hot worked strip.

5. Apparatus as claimed in Claim 4 wherein the powder compaction means comprises a compaction mill including a pair of compaction rolls mounted with their axes of rotation located in a common horizontal plane.

6. Apparatus as claimed in Claim 4 or Claim 5 wherein the shot impaction means comprises a sealed shot impaction unit including a pair of shot impellers driven by motors and supplied with shot from feeders.

7. A method for the continuous production of metal strip substantially as herein described with reference to the accompanying drawings.

8. Apparatus for the continuous production of metal strip substantially as herein described with reference to the accompanying drawings.