WO2005016565A1

WO2005016565A1 - Cryogenic descaling process

Info

Publication number: WO2005016565A1
Application number: PCT/CA2004/001519
Authority: WO
Inventors: Barry A. Herring; John Fitzgerald Herring; Sundaresa Venkata Subramanian
Original assignee: Lec Technologies Inc.
Priority date: 2003-08-18
Filing date: 2004-08-17
Publication date: 2005-02-24

Abstract

The invention provides a process for descaling metal comprising applying cryogenic coolant to the metal in conjunction with a mechanical descaling operation to facilitate the removal of wet mill scale from a surface of the metal and condition the metal surface for application of lubricant for cold working. The metal may comprise hot-rolled wire rod and the mechanical descaling operation may include at least one of a reverse bending operation; a brushing operation; and a blasting operation. In one embodiment the cryogenic coolant is applied prior to the mechanical descaling operation as a bath immersion or stream injection in order to produce a dry rod, free of wet mill scale. In a further embodiment, the cryogenic coolant is applied as a cryogenic jet or blast after the mechanical descaling operation in order to remove fine oxide particles and moisture, thereby preventing them from contaminating the lubricant used for cold working of wire rods.

Description

CRYOGENIC DESCALING PROCESS

TECHNICAL FIELD

The invention is directed to the surface treatment of metals and more particularly to descaling as-rolled hot-rolled wire rod.

BACKGROUND OF THE INVENTION

Steel, when treated at high temperatures and in contact with oxygen in the air, forms an iron oxide scale on its surface. The oxide scale is commonly referred to as mill scale in the literature [The Making, Shaping and Treating of Steel, 10^th Edition, U.S. Steel, Ed. William T. Lankford et al., Reproduced by Association of Iron and Steel Engineers, 1985,section on Oxide removal, page 1083.] Often to further process the steel, a descaling process must be undertaken. Scale on hot-rolled wire rod may comprise between 0.25 to 2 percent of the product by weight. The traditional chemical method to remove the mill scale by acid pickling has become an environmentally undesirable process. Therefore descaling by mechanical means has become the preferred method.

One method of mechanical descaling includes subjecting the hot-rolled wire rod of a typical diameter of between 5mm and 19mm to a plurality of bending operations^' in different directions. Preferably, the bending occurs along orthogonal planes.

Additionally, mechanical descaling may comprise brushing operations (with wire brushes, or other media such as steel wool or tumbling media), blasting operations (using forced air, shot, or scale), or a combination of such operations as described in Canadian Patent No. 1075911 granted to Tefilunion S.A.

One disadvantage of mechanical descaling is that the product to be descaled should be dry before commencing such operations in order to achieve production speeds, minimize lubricant contamination, and avoid excessive wear of equipment and tooling. Water on the surface of the product tends to occlude broken scale to the surface. If not removed, water and/or occluded scale can interfere with other operations such as lubrication and subsequent processing. Hot-rolled wire rod is often lubricated with a solid or liquid-based lubricant before being further processed for size. The surface water and/or broken scale may contaminate the lubricant, causing waste, ineffective lubrication and poor product quality.

A wash treatment may be optionally applied following mechanical descaling to remove fine particles and dust prior to further processing. Disadvantages to washing include the need to consume water and deal with the waste water to avoid environmental contamination. A further disadvantage is that the wire typically must be dried before further processing or storing.

Hot-rolled wire rod and other such metal products are often stored uncovered and outdoors before processing. This may be due to the size of the product and inventory carried by those processing the product or, for those employing a just-in-time manufacturing process, because the product is often recently transported for receipt just prior to processing. Product stored or transported in a natural environment is often wet. To dry it out, it is brought in to a dry environment for storing before processing. Costs for storing the product indoors can significantly contribute to the cost of processing and thus affect profitability and process scheduling.

U.S. Patent No. 5,395,454 of Robert issued March 7, 1995 describes a method of cleaning elongated objects (consisting of wires, rods, tube-shaped objects and flat-shaped objects or combinations thereof) of surface contaminants. The method comprises exposing the elongate object to a gaseous, liquid or solid inert gas or any combination thereof in an amount sufficient to embrittle the contaminants whereupon the elongated object is drawn through an orifice (e.g. a die) causing the embrittled surface contaminants to be removed from the object.

The mill scale produced at the Rod Mill is brittle, hard and has a coefficient of friction, when presented to other metallic surfaces. Scale should therefore be removed before cold drawing and cold working of any sort is undertaken. (Steel Wire Handbook, vol. 1, ch. 3, The Wire Association, Inc., p. 93). Regardless of the amount and nature of the scale, it must be completely or very nearly completely removed prior to the drawing if a quality wire is to be produced. (Steel Wire Handbook, vol. 1, ch. 3, The Wire Association, Inc., p. 114). Thus, wire rods ought to be relatively free of mill scale and lubrication ought to be used to assist with the wire drawing operation. Sizing a die to the section of the wire to only allow the passage of the wire and not its mill scale is inherently difficult, particularly when it comes to removal of mill scale normally encountered in hot rolled wire rod surface. Moreover, rather than flaking off at the die, a substantial portion of the scale gets drawn -into the die, resulting in wear and changes to the shape of the die. The drawn wire is thus not uniform and the die wears out prematurely.

Therefore, it is desirable to provide a process for descaling that is effective for wet product.

A solution to one or more of these shortcomings is therefore desired.

SUMMARY OF THE INVENTION

A process for descaling metal comprises applying cryogenic coolant in conjunction with a mechanical descaling operation to facilitate the removal of wet mill scale from a surface of the metal and condition the metal surface for application of lubricant for cold working. The metal may comprise hot-rolled wire rod and the mechanical descaling operation may include at least one. of a reverse bending operation; a brushing operation; and a blasting operation. In one embodiment the cryogenic coolant is applied prior to the mechanical descaling operation, for example, as a bath immersion or stream injection in order to produce a dry rod , free of wet mill scale. In a further embodiment, the cryogenic coolant is applied as a cryogenic blast after the mechanical descaling operation in order to remove fine oxide particles and moisture, thereby preventing them from contaminating the lubricant used for cold working of wire rods.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:

Fig. 1 illustrates schematically a production line for hot-rolled wire rod including a cryodescaling operation in accordance with an embodiment of the invention;

Fig. 2 is an image of a surface of a length of wire rod following descaling a dry wire rod dried by conventional storage inside a shop; and Fig. 3 is an image of a surface of a second length of wire rod following descaling of a wet wire rod subjected to cryogenic treatment.

It will be noted that throughout the appended drawings, like features are identified by like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Fig. 1 illustrates schematically a production line 10 for working hot-rolled wire rod (not shown) in accordance with an embodiment of the invention to remove water and facilitate mechanical descaling. Broadly, production line 10 includes a mechanical descaling stage 12 and a processing stage 14. Hot-rolled wire rod typically comprises a coil and those of ordinary skill in the art will appreciate that not shown in Fig. 1 are an unwinding stage and, usually, a re- winding stage which facilitates the moving of the wire through the production line 10.

Mechanical descaling stage 12 comprises a plurality of mechanical descaling devices, namely a reverse bending device 16, a brushing device 18 and an air wiping device 20 in series as indicated by arrows from left to right. Reverse bending devices are well known to those of ordinary skill in the art, comprising a series of rollers (not shown) to bend the wire, preferably in two planes that are at right angles to one another, to break the scale from the surface of the wire.

Brushing devices are also known and comprise one or more stiff brushes (e.g. wire) (not shown) arranged to contact the surface around the wire to further remove scale from the wire, including scale broken by bending device 16. Air wiping device 20 provides forced air to the surface of the wire to remove fine particles of scale and/or dust that may remain adhered to the wire. Persons skilled in the art will appreciate that one or more of devices 16, 18 and 20 may be contained in a common housing for collecting broken scale and/or dust. Though exemplary line 10 comprises three descaling devices, it is understood that alternate configurations, including alternate devices (e.g. such as shot blasting or (belt) sanding devices), may be employed.

Further processing is performed at processing stage 14 which comprises a lubricant device 22, for example, a hopper box (not shown) containing lubricant powder, through which the wire may be advanced and a cold-working apparatus 24 such as for reducing the dimension of the wire. While preferred, the performance of such further processing operations need not occur immediately following the descaling stage.

In accordance with the invention, production line 10 for cryodescaling comprises at least one cryogenic coolant application device 26 for applying an extreme cold coolant to the wire at one or more points in the line 10 as it is advanced therethrough. Exemplary line 10 includes a single device 26 for applying a cryogenic coolant in association with the mechanical descaling stage 12, such as at one or more of a point prior to bending device 16 (illustrated by dashed line 27) and after air wiping device 20 (illustrated by dashed line 28). Though shown in-line with air blasting device 20, it is understood that the application of cryogenic coolant at line 28, as discussed further below, may replace air wiping via device 20.

Device 26 may comprise a cryogenic coolant source such as a coolant store of liquid nitrogen or other cryogenic (e.g. liquid helium and others well known to persons of ordinary skill in the art) and a supply such as a line for delivering the coolant to the surface of the wire. Liquid nitrogen is relatively inexpensive and inert and thus may be preferred in some embodiments. Application may be combined with a mechanical force, blasting the coolant over the wire, such that the process incorporates cryogenic and mechanical energies. The liquid nitrogen or other fluid stored under pressure in device 26 may be released in a controlled manner over the traveling wire. The coolant need not be in a liquid foπn but may be in a solid or gaseous state as is well known to persons of ordinary skill in the art. One or more trials may be required to determine an appropriate coolant and coolant form as well as the amount to be applied to the wet object in order to remove the water therefrom.

Though it is illustrated that the application of the coolant is performed in series with mechanical descaling operations, for example, at the beginning or end of the stage, the application may be performed contemporaneously, for example, while the wire is undergoing bending operations. Hence, application of cryogenic coolant to the metal in conjunction with a mechanical descaling operation may take many varied forms.

Application of coolant before mechanical descaling, as illustrated by line 27, removes or assists with the removal of surface water from wet hot-rolled wire rod to facilitate mechanical descaling. Experiments indicate wet wire processed in this manner exhibits comparable properties to dry wire processed without the application of a cryogenic coolant.

A series of laboratory and production line experiments were performed on lengths of wire rod. In the laboratory, rods in the dry and wet condition were studied for two modes of application, namely liquid nitrogen applied by bath immersion and stream injection at 5-10 pounds per square inch. Experimental observations show that the descaling of a length of wet rod subjected to mechanical descaling by simple bending is limited compared to a bench mark comprising a dry rod subject to the same mechanical descaling. However, the descaling of a length of wet rod subjected to a blast of liquid nitrogen is comparable to the bench mark case and the descaling of a length of wet rod subjected to a bath of liquid nitrogen prior to mechanical descaling approaches the bench mark case.

In a production line configured as per Fig. 1 with application of a cryogenic coolant as per line 27, experiments were performed on dry and wet rods. Fig. 2 is an image obtained by scanning electron microscope showing the surface of a sample of a length of wire processed from dry hot-rolled wire rod in accordance with the prior art (i.e. without activating device 26 for the application of a coolant). Similarly Fig. 3 is an image obtained by scanning electron microscope showing the surface of a sample of a length of wire processed from wet hot-rolled wire rod in accordance with the invention where device 26 was activated and coolant applied prior to bending operations. From the appearance of the surface of the samples, comparable scale removal is apparent as illustrated by the Figs.

Experimental observations show that the surface of the wire is dry when exiting the descaling stage (proximate to the application point at line 28).

Additionally, in the production line as per Fig. 1 with application of cryogenic coolant as per line 28, experiments were performed on lengths of wire. Experimental observations indicate that traveling wire cleaned by cryogenic blast is cleaner than wire cleaned by air wipe alone. Further, occlusion of particles in liquid nitrogen before they become air-borne is an added advantage in making the process environmentally friendly. The occluded particles may be more easily directed for particle collection. The application of cryogenics to a wet rod is found to facilitate descaling of wet mill scale by mechanical descaling and subsequently assist in the application of lubricant on to the surface for cold working of the wire rod.

The embodiment(s) of the invention described above is(are) intended to be exemplary only and persons of ordinary skill in the art will appreciate that variations and modifications may be made thereto without departing from the scope of the invention. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.

Claims

WE CLAIM:

1. A method for descaling wet mill scale from hot-rolled wire rod comprising: applying a cryogenic coolant to the wet mill scale to enable a mechanical descaling operation.

2. The method as claimed in claim 1 wherein the cryogenic coolant is applied as one or more of a cryogenic bath immersion, liquid stream injection, jet and blast.

3. The method as claimed in claim 1 wherein the mechanical descaling operation includes at least one of a reverse bending operation; a brushing operation; and a wiping operation.

4. The method as claimed in claim 1 wherein the cryogenic coolant is applied at least one of prior to and after the mechanical descaling operation.

5. The method as claimed in claim 1 wherein the applying is performed in-line in a production for working the wire rod.

6. The method of claim 1 comprising applying the cryogenic coolant to remove broken scale occluded to the wire rod following a mechanical descaling operation.

7. A method of preparing an elongate metal for processing in a production line comprising: in the production line, applying a cryogenic coolant to the elongate metal sufficient to remove from a surface of the elongate metal at least one of: a) wet mill scale; and b) occluded matter remaining from a processing operation to remove the matter in the production line.

8. The method of claim 7 wherein said applying removes occluded matter comprising broken mill scale remaining from a mechanical descaling operation in the production line and conditions the surface for application of lubricant for cold working.

9. A method for descaling hot-rolled wire rod comprising: applying a cryogenic coolant as a cryogenic blast to the hot-rolled wire rod in conjunction with a mechanical descaling operation to facilitate the removal of wet mill scale from a surface of the hot-rolled wire rod and condition the surface of the wire rod for the application lubricant for cold working; wherein the mechanical descaling operation includes at least one of a reverse bending operation; a brushing operation; and a blasting operation; and wherein the cryogenic coolant is applied at least one of prior to and after the mechanical descaling operation.