US3525507A - Fluidized-bed system for patenting steel wire - Google Patents

Description

Aug. 25, 1970 ET AL 3,525,507

FLUIDIZED-BED SYSTEM FOR PATENTING STEEL WIRE Filed Oct. 16, 1967 5 Sheets-Sheet 1 3o 40 50 6o 80 0o 11sec] FIG I H. Geipel E.. Fb'rsfer W. Heinemann INVENTORS.

WA R088 Attorney Aug. 25, 1970 H, q pE ET AL 3,525,507

FLUIDIZED-BED SYSTEM FOR PATENTING STEEL WIRE Filed Oct. 16. 1967 3 Sheets-Sheet 2 INVENTORS HANS GE/Pfl. :cxemnar Fozsma warn/0 HE'INEMANJV Kol 'A rramvrr Aug. 25, 1970 H. IGEIPEL ET AL FLUIDIZED-BED. SYSTEM FOR PATENTING STEEL WIRE Filed Oct. 16. 1967 3 Sheets-Sheet 5 FIG. 3

1N'VENTORS HANS GE/PEL, sake-W420? #025724 wa e/Ea I/E/NEMAN/V I. 6R9 I l ,4 Tran/v45? United States Patent 3,525,507 FLUIDIZED-BED SYSTEM FOR PATENTING STEEL WIRE Hans Geipel, Oberhausen-Sterkrade, Eckehardt Forster,

Oberhausen, and Wilfried Heinemanm Duisbur'g, Germany, assignors to Huttenwerk Oberhausen AG, Oberhausen, Germany, a corporation of Germany Filed Oct. 16, 1967, Ser. No. 675,522 Claims priority, application Germany, Oct. 25, 1966, H 60,847, H 60,848 Int. Cl. C21d 9/56 US. Cl. 266-3 6 Claims ABSTRACT OF THE DISCLOSURE Austenitic steel wire, having a starting temperature above the transformation point Ac is passed through a fluidized bed of solid ceramic particles entrained by a stream of carrier gas, the gas temperature and/or the residence time in the bed being so chosen that the wire emerging from the bed has a temperature (around 550 C.) in the range in which transformation to sorbite occurs but above the level (around 500 C.) below which the formation of bainite would take place.

Our present invention relates to a plan for patenting steel wire.

The term patenting is understood in the wire-making art as identifying a treatment of hot steel wire of medium or high carbon content, preparatory to a drawing of the wire to final size, whereby the temperature of the wire is reduced at a controlled rate from a level above the critical point Ac (transformation of ferrite to austenite) to a range in which austenite is transformed into pearlite. This cooling is conventionally carried out in air or in a fused bath, such as molten lead.

In order to satisfy the usual requirements of ductility, flexibility and tensile as well as torsional strength, the wire so treated should have a predominantly sorbitic crystal structure. Sorbite is a fine-grained variant of pearlite and comes into existence upon transformation of austenitic steel at a temperature of approximately 550 C. If the transformation occurs at a lower level, generally below 500 C., the pearlite crystals are still smaller and form a structure known as bainite. This structure is consideraby harder than the sorbite and unsuitable for drawing. If, on the other hand, transformation is allowed to occur at temperatures above the level of substantially 550 C., the pearlite becomes progressively coarser as its crystals are surrounded by a ferrite skeleton; such a wire, typically obtained by patenting in air, has good ductility and torsional strength but does not withstand flexure as well as does wire transformed in a range of about 500 to 550 C.

The general object of our invention is to provide means for so cooling a steel wire, previously heated above the critical range, that the desired sorbitic structure is invariably obtained in a rapid and economical manner with substantial exclusion of bainite.

A more particular object of this invention is to provide a treatment of this type which can be applied directly to wire coming hot from a rolling mill.

In accordance with our present invention, a hot wire of austenitic steel is continuously passed through a cooling medium of the fluidized-bed type, i.e. a stream of carrier gas with entrained solid particles such as ceramic granules of elevated heat-transfer coefiicient (preferably between about 500 and 1000 Cal./m. /hr. C.) The particles may consist, for example, of magnesia and may range between 0.03 and 0.15 mm. in diameter, with a bulk weight of 1.5 to g./cm. Hydrogen, carbon mon- 3,525,507. Patented Aug. 25, 1970 oxide or other relatively inert gases conventionally used in metallurgical processes may serve as the carrier fluid. Though the temperature of the cooling medium (solid particles and carrier gas) may be well below the bainiteformation level of about 500 C., transformation is completed above that level because the wire is led out of the fluidized bed in a state of incipient transformation before its temperature falls below the 500 C. mark. It will be understood that the exact temperature range to be observed for optimum results varies somewhat with the composition of the steel and can be determined from available handbooks.

We have found that the treatment of wire by our present method results in a sorbitic structure comparable to that realizable, albeit at substantially lower production rates, with a bath of molten lead. Moreover, the treatment according to our invention is faster than patenting in air and tends to suppress the formation of the ferrite skeleton usually associated with air cooling.

After the wire has emerged from the fluidized bed, transformation proceeds to completion under substantially isothermic conditions, i.e. without the use of a cooling medium other than the surrounding atmosphere. To retard the cooling at this stage it is, however, desirable to shield the emerging wire by sheet-metal plates or the like reflecting its thermal radiation. The final cooling, subsequent to transformation, may also take place in air.

In order to stabilize thetemperature of the emerging Wire within the desired range of approximately 500 to 550 C., we prefer to measure that temperature and to compare it with a predetermined value to compensate for deviations therefrom by a corrective adjustment of the bed temperature and/ or of the residence time of the wire in the fluidized bed. To control the temperature of the cooling medium, we prefer to remove particles continuously from the bed and to let them pass through a cooling chamber before returning them to the bed; this recirculation of the particles is best accomplished with the aid of a flow of carrier gas which may itself be recirculated.

A plant suitable for carrying out the aforedescribed method comprises a conveyor, preferably in the form of an apertured belt, passing through a channel together with the stream of carrier gas and entrained solid particles; the discharge end of the channel is provided with a gate through which the cooled wire may emerge while the particles are retained and form a nearly stationary accumulation around the exiting wire. The hot incoming wire may be deposited on the conveyor in a succession of loops, advantageously with the aid of a transversely oscillating dispenser as disclosed and claimed in our commonly owned application Ser. No. 675,405 filed on even date herewith under the title Method of and Means for Cooling Wire.

The invention will now be described in greater detail with reference to the accompanying drawing in which:

FIG. 1 is a transformation diagram showing the conversion of austenitic steel to sorbite by conventional means and by the process of our invention;

FIG. 2 is a somewhat diagrammatic side-elevational view of a plant for carrying out the process; and

FIG. 3 is a fragmentary view similar to FIG. 2, showing a modification.

In FIG. 1 we have shown at A and B the boundaries of the austenite/pearlite transformation range for a typical steel wire of 5.5 mm. diameter, made from unalloyed steel with a carbon content of 0.5%. Graph e represents an idealized process whereby the wire is rapidly cooled, from a starting temperature of 860 C., to a level of 550 C. which it reaches after 1 /2 seconds and where the graph intersects the boundary curve A of the transformation range. After a further interval of about 18 /2 seconds, with gradual cooling to a point at or about 500 C., the transformation to sorbite would be completed without the formation of appreciable quantities of bainite. Such an idealized cooling process, e.g. with quenching in water, would be difficult to realize because of the problems of temperature control and appears to be impractical for any but the thinnest wires.

It is widely assumed, even if not established by incontrovertible proof, that the qualities of steel wire especially in regard to fiexure are improved by an approximation of the conditions represented by graph e. This may be accomplished, to a certain extent, by the use of a batch of molten lead (graph a) which, in order to avoid the formation of bainite, should be maintained at a temperature of about 500 C. so that the curve approaches this level asymptotically; this type of treatment, completed after 20 seconds, does not lend itself to the processing of hot wire coming at relatively high speed from a rolling mill. Conventional air cooling (graph d) takes even longer and leads to incipient transformation at a temperature close to 700 C., with resulting formation of a large-grain ferrite structure in the pearlite.

The patenting of wire in accordance with our present invention is represented by graphs b and 0. Graph b illustrates the cooling by ceramic granules of the aforedescribed type having a heat-transfer coefficient oc=600 Cal./m. /hr./ C. as compared with a value 01:1180 for the lead bath of graph a. Graph c applies to ceramic particles with a=850. The particle temperature is maintained well below 500 C., yet contact between the particle stream and the wire is terminated at a point p or q, thus after 8 or 6 seconds, respectively, when the wire temperature drops to a level of 520 C. The treatment then continues substantially isothermally for a further period of approximately 20 seconds, to a point r well beyond the intersection of graphs b and c with curve B, whereupon final cooling proceeds in the open air (without any thermal shielding) as indicated by the joint portions b, c of the two graphs.

Reference will now be made to FIG. 2 for a description of a plant in which the process described in connection with FIG. 1 can be performed. The plant comprises a fluidized bed 1 confined within a tunnel 24, forming an elongated flow channel, to the vicinity of the upper run of an endless conveyor belt 2 which is continuously driven by a motor 15 so that a hot wire 3 deposited thereon is transported on a downwardly sloping path from right to left. Wire 3 passes through a guide tube 4 and a continuously rotating dispenser arm 25, driven by a motor 26, whose rotation forms the wire into a succession of loops deposited on the conveyor 2; the dispenser arm 25 may be subject to continuous transverse oscillations at a frequency related to the loop-deposition rate, as described in our above-identified copending application of even date, for the purpose of insuring optimum distribution of the loops over the available conveyor surface. Belt 2, designed as a wire screen or other apertured member, transports the loops through a gate 8 at the discharge end of the channel, this gate being here shown as a simple shutter having a slot for the passage of the wire loops; a more elaborate gate, designed to prevent the loss of solid particles through the exit slot, has been disclosed in our concurrently filed and commonly owned application Ser. No. 675,426 entitled Fluidized Bed, now abandoned. A perforated base 27 within tunnel 24 forms the lower boundary of bed 1 and is connected to outlets of a manifold 10 through which a carrier gas, as indicated by the arrows, is passed at longitudinally spaced locations by way of the interstices of belt 2 into the space thereabove. The branch conduits of manifold 10 contain respective valves 9 for controlling the amount of gas thus introduced. A further valve 28 controls the input from a compressor or other high-pressure source, not shown, whereas two other valves 29, 30 determine the proportion in which a portion of the gas is branched off into a conduit 5 into which opens an outlet of a cooling chamber 6, the latter containing a coil 22 traversed by a coolant. Conduit 5 opens into the tunnel 24in the vicinity of the housing 23 of the dispenser arm 25.

Solid particles entrained by the gas stream accumulate in a pile just ahead of the shutter 8 where the tunnel 24 is formed with a discharge port 7 for these particles. A similar accumulation is formed at the entrance end of the tunnel by means of a stationary plate 31 underlying the upper run of conveyor belt 2 beneath an inlet branch 32 of conduit 5. Port 7 communicates with a further conduit 33 which leads to the top of cooling chamber 6 and which may include means, such as a pump 34, to promote the return of solid particles from the discharge end of tunnel 24 to the cooler. Another conduit 16, provided with a control valve 35, serves as a suction line to exhaust particles from the vicinity of shutter 8 to a separator 18 when they are returned to cooled 6 via a pipe 21; the spent carrier gas drawn off by line 16, and by a "branch 36 thereof extending from the entrance end of the channel, is removed by a pump 17 into a conduit 19 whence it may be discharged by way of a valve 37 to the atmosphere or to the low-pressure side of the compressor delivering fresh gas to valve 28. A bypass 20, controlled by a valve 38, enables the recirculation of some or all of the gas to manifold 10.

In accordance with an important feature of our invention, a temperature feeler 11 just beyond shutter 8 senses the temperature of the emerging wire loops and feeds this information to a comparator 13 receiving a reference signal from a storage device 12 adjusted to the desired exit temperature (e.g. 520 C.). Comparator 13 sets a controller 14 which, if necessary, adjusts the speed of motor 15 to vary the residence time of the wire in the fluidized bed 1 in a manner compensating for any deviations of its exit temperature from the preset reference value.

Dispensing arm 25 is, of course, representative of any convenient type of loop depositor including, for example, devices of the type shown in US. Pats. Nos. 3,056,433 and Re. 26,052.

The -wire 3 exiting from gate 8, thermally shielded against excessive radiant-heat losses by a tube 39 forming an extension of tunnel 24, continues on conveyor 2 in the ambient atmosphere until its transformation has been completed (point r in FIG. 1). Thereafter, it may be aircooled more rapidly outside the tube 39, by the same or another conveyor or without any conveyor at all, to room temperature.

In FIG. 3, where elements corresponding to those of FIG. 2 have been designated by the same reference numerals with addition of a prime mark, we have shown the temperature sensor 11 disposed ahead of shutter 8'. Sensor 11' ascertains the exit temperature of the wire in terms of the temperature of the fluidized bed 1 at the discharge end of tunnel 24' and, as before, communicates this information to a controller 14'; the output of this controller, in contradistinction to the previous embodiment, sets a servomotor 40 which adjusts a valve 41 to regulate the amount of cooling fluid passing through coil 22 of chamber 6'. The system operates otherwise in the same manner as the arrangement of FIG. 2. Naturally, the control systems 11, 11 shown in FIGS. 2 and 3 could also be combined in a single plant.

We claim:

1. A plant for patenting steel wire, comprising a continuously movable apertured conveyor belt; dispenser means for continuously depositing upon said conveyor belt a length of austenitic wire heated above the critical temperature range; circulating means for blowing along said conveyor belt a gas stream with entrained solid particles; channel means for confining said gas stream to the vicinity of said conveyor means, said channel means including a base below said belt having perforations spaced in the direction of belt motion and connected to said circulation means for admitting carrier gas through the apertures of said belt into the space thereabove; gate means at a discharge end of said channel means for forming a terminal accumulation of said particles through which said wire passes on emerging from said gas stream; and control means for so regulating at least one operating parameter of the plant as to maintain the temperature of the emerging wire substantially at a level of incipient transformation of the austenite to sorbitic pearlite but above a point below which substantial amounts of bainite would be formed.

2. A plant as defined in claim 1 wherein said control means comprises temperature-sensing means at said discharge end and speed-regulating means for said conveyor belt responsive to said temperature-sensing means.

3. A plant as defined in claim 1 wherein said control means comprises temperature-sensing means at said discharge end and temperature-regulating means for said particles responsive to said temperature-sensing means.

4. A plant as defined in claim 1 wherein said conveyor belt extends generally horizontally with a downward slope toward said discharge end.

5. A plant as defined in claim 1, further comprising thermal shield means beyond said discharge end for limiting heat radiation from the emerging wire to the surrounding atmosphere.

6. A plant as defined in claim 1 wherein said circulation means includes a suction line for drawing spent gas from said channel means, separator means in said suction line for removing entrained particles from said spent gas, means for cooling particles removed from said separator means, and conduit means connected to a source of carrier gas under pressure for returning the cooled particles from said chamber to said channel means.

References Cited UNITED STATES PATENTS 3,181,977 5/1965 Sturgeon 2664 X 3,252,693 15/1966 Nelson 2663 3,355,159 11/1967 Ayers 2663 3,390,871 7/1968 McLean et a1. 2663 3,391,915 7/1968 Morgan 2663 20 I. SPENCER OVERHOLSER, Primary Examiner R. S. ANNEAR, Assistant Examiner US. Cl. X.R.

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