WO1991011541A1

WO1991011541A1 - Inline galvanising process

Info

Publication number: WO1991011541A1
Application number: PCT/AU1991/000025
Authority: WO
Inventors: Paul Raymond Kelly; Brian Roy Crossingham
Original assignee: Tubemakers Of Australia Limited
Priority date: 1990-01-25
Filing date: 1991-01-23
Publication date: 1991-08-08

Abstract

A method of producing a hot dip metal coated structural element from continuous steel strip. The method having the steps of shot blasting the steel strip; forming the steel strip into a structural element, pickling and then heating the structural element; passing the structural element through a liquid metal bath then through a coating control device and then cooling and/or quenching said structural element.

Description

INLINE GALVANISING PROCESS

Technical Field

The present invention relates to a continuous process for the production of structural sections from hot rolled steel strip having improved surface characteristics and improved corrosion protection.

Background

Traditional production techniques for structural sections, whether open or closed, involve forming of the desired shape from hot rolled steel strip. This is generally followed by the application of a temporary protective coating, either clear or pigmented, over the top of the residual mill scale on the section.

Following subsequent fabrication, longer term corrosion protection for the completed structure is achieved by removal of the scale layer and recoating with appropriate paint systems. Alternatively, the completed sections may be batch galvanised, thereby affording the well-known benefits of the metallurgically-bonded zinc coating, i.e. superior corrosion protection, toughness, reduced maintenance and enhanced appearance.

In either case, these final enhancement and protection processes are time consuming, labour intensive and expensive. Even in the case of galvanising, the inherently heavy coatings produced by the conventional batch process often militate against its use due to cost considerations. Significant benefits would arise if galvanised coatings could be applied to structural members with a thickness similar to those normally found on sheet metal.

Disclosure of Invention

The fundamental purpose of the present invention is to provide a method for the continuous processing of hot rolled steel strip into structural sections, incorporating the steps of scale removal and corrosion protection through the application of galvanised coatings with a thickness similar to those normally achieved with sheet metal such as that used for roof, wall and other exterior uses.

The method is not limited solely to the application of zinc coatings and it will be recognised that the same techniques can be used to apply other metallic coatings to steel sections, (e.g. Zn - Al, Al, Zn-Ni, etc).

In one broad form, the present invention is a method of producing a hot dip metal coated elongate structural element from a continuous steel strip comprising the steps of:

(a) shot blasting the steel strip;

(b) forming the thus shotblasted steel strip into a structural element of a desired cross section;

(c) pickling said formed structural element;

(d) heating said structural element to a suitable temperature range;

(e) passing said structural element through a liquid metal bath, thereby coating said structural element with a liquid metal;

(f) passing said structural element through a coating control device to remove excess coating; and

(g) passing said structural element through a cooling and/or quenching station.

In a further form the present invention is a method of producing a galvanised elongate hollow structural section from steel strip in a continuous process comprising the steps of:

(a) shotblasting the steel strip;

(b) forming the thus shotblasted steel strip into a substantially circular cross section with an open seam;

(c) passing the substantially circular cross section through a welding station and welding of the open seam thereby forming a hollow section; (d) pickling the thus formed hollow section;

(e) rinsing the hollow section and applying a flux coating of zinc chloride and ammonium chloride to said hollow section;

(f) heating said hollow section to a desired temperature range;

(g) passing said hollow section through a galvanising bath thereby coating said section with a zinc/zinc alloy coating;

(h) passing the thus galvanised hollow section through a coating thickness control device; and (i) passing the galvanised hollow section through a quenching and/or cooling station. The invention will now be described by way of a non-limiting example with reference to the accompanying drawings in which:

Brief Description of Drawings

Fig. 1 is a line diagram of a mill for the continuous forming and galvanising of heavy gauge hollow sections from steel strip.

Fig. 2 is a sectional elevational view of the coating device.

Mode for Carrying Out Invention

With reference to Fig. 1, coils of hot rolled steel strip 2 are taken from coil storage and loaded into a coil feed magazine 3. The coil feed magazine 3 sequentially transports the coils of steel strip 2 to an uncoiling station 4, which consists of an uncoiling mandrel (or mandrels) with a means of peeling away the leading end of each coil and guiding it into the next stage of the process.

In the next stage the leading edge of the coil strip 2 coming from the uncoiling station 4 is fed into a pinch roll 5, followed by a set of leveller rollers 6, the purpose of which is to flatten the strip 2 and remove any coil set induced by previous processing operations. Once the strip has passed through the leveller rollers 6, it is brought to a splice welding station 7, where the tail end of the previous coil strip which has gone through the line and the leading end of the new coil strip are sheared and welded together. The ends of these adjacent coils may be sheared sequentially, or simultaneously by means of a double shear.

The coil welding operation may be accomplished by techniques including flash welding, "stick" or electrode welding, or continuously fed electrode welding with inert gas shielding.

The aim of the welding techniques is to provide a soundly prepared joint between the coils of steel strip 2, in a time and with a continuity which is compatible with the capacity of the forming and processing mill as a whole.

Once the joint has been prepared, the resulting weld upset may or may not be scarfed from both, or only one of the surfaces at the discretion of the operator or as required by product end usage.

After splicing, the steel strip 2 is conveyed via another pinch roll 9 into an accumulation system 10, the purpose of which is to maintain a reservoir of the steel strip 2 and having sufficient capacity to allow continuous operation of the subsequent processing steps, under normal operating conditions.

The nature of the accumulation system 10 can include: a looping pit with vertical strands of steel strip 2 between telescoping sets of rolls, a horizontal looping car either under or over the mill line, or a spiral accumulator mounted either vertically or horizontally. A spiral accumulator is more suitable for heavier gauges encountered in structural sections and is more efficient in terms of space, cost and ongoing maintenance.

The steel strip 2 exits the accumulation system 10 and passes continuously through a shot blast station 11, which constitutes the first stage in the process of preparing the surface of the steel strip 2 for the later zinc/zinc alloy coating. In the shot blast station 11, abrasive material is impacted on the strip surface, with a force and intensity sufficient to remove the mill scale formed during previous hot processing leaving an essentially clean surface. Precise control of shot size and force is most important so as to avoid the occurrence of an unsatisfactory heavily indented surface with subsequent implications for quality of the product surface appearance and for excessively thick zinc/zinc alloy coating. Excessive force can also stretch the strip, changing its shape away from the optimum profile.

Shot blast preparation may be applied to both, or only one of the strip surfaces at the discretion of the operator, or as dictated by product end-usage.

As steel strip exits the shot blast station 11 it continuously passes into a shape preparation machine 12 where any necessary forming operations can be carried out on the prepared strip 2. Further forming operations may be performed after the later zinc/zinc alloy coating stage.

In the preferred embodiment of the invention, the steel strip 2 is formed into a longitudinally closed section, approximately circular in shape. To achieve this the prepared strip is passed into sets of forming rolls which progressively change its shape from the original flat profile into an approximately circular form with an open seam. In the final few roll stations of the shape preparation machine 12, special blades incorporated within the rolls are interposed between the strip 2 edges to prepare them for the subsequent seam welding operation.

Immediately prior to welding, the section passes through an electric induction coil which induces a current flow in the section. Assisted by the influence of ferrite impeders located inside the section, the induced current flows around the circumference of the section, however at the open seam it flows along the strip edges, which are caused to converge by a set of welding rolls located downstream of the induction coil. The closing edges complete the current path, while the passage of the current along the surfaces of the converging strip edges heats the surface, resulting in fusion of the strip edges as they are brought together in the welding rolls. At this point in the preferred embodiment of the invention, the steel section 16 (formed from the steel strip 2) is essentially tubular in shape.

As part of this section of the process, it is normal practice to provide a means of cooling the ferrite impeders located inside the section, to maintain efficient welding conditions. This cooling may be carried out using air, gas or liquid at ambient or non-ambient temperatures and may incorporate techniques to prevent or minimise the amount of such coolant being carried inside the steel section further along the production line.

Again when forming closed sections by welding, devices may be used to scarf or flatten excess metal from or located on the external and/or internal surfaces of the steel section. In the preferred embodiment, a scarfing tool is provided downstream from the welding rolls to remove external excess metal.

Where the preforming operation results in heating of the section (such as would occur in longitudinal seam welding) , a cooling section 8 is interposed between the preforming operations and the next processing step. In the preferred embodiment, the closed section passes continuously beneath sets of sprays which cause cooling liquid, generally water, to flow over the section.

The amount of cooling is adjusted to ensure that the section temperature is optimized relative to the chemical process performed in the next stage of the process.

Subsequent to the pre-forming step, the steel section then passes continuously through an acid-pickling stage 13. In this section, the surface of the steel section is contacted with an acid medium which further chemically prepares the section surfaces, to remove any oxides, dirt or other material which would otherwise interfere with the zinc/zinc alloy coating process. Where heating has occurred subsequent to shot blasting removal of resultant oxides would be an important consideration.

The pickling contacting method can include mist, spray or immersion techniques. A variety of acid media could be used, such as sulphuric or hydrochloric acid, and preheating of the acid media could be employed to enhance the pickling action. In the preferred embodiment, an aqueous solution of approximately 13% hydrochloric acid is caused to flow over the moving steel section, using suitably located sprays.

A system of weirs and air knives is used to prevent carry over of excess acid solution to subsequent operations. ,

After acid pickling, the treated steel section passes continuously through a rinsing stage 14, where water is used to remove residual acid solution. Whilst this step is not mandatory, it is recommended. During rinsing, immersion is used, however, spray or mist techniques could also be employed.

Subsequent to acid pickling (and rinsing) excess liquid is removed by air knives 15 in the current example and the section passes continuously through a fluxing tank 17, in which a solution of zinc chloride and ammonium chloride is applied to the surface of the section. Fluxing is an optional step and is preferable rather than essential. The primary purpose of this coating is to maintain the chemical cleanliness of the steel surface prior to contacting with molten zinc.

Following removal of excess flux solution by air knives 15, the treated steel solution passes continuously into a heating apparatus 18, which may also feature an inert or reducing gas atmosphere. In the current example, an inert gas atmosphere is employed. Again, while different heating techniques can be used such as radiant electric, radiant gas or electric induction, the latter method is employed in the preferred embodiment.

It should be noted that subsequent to acid pickling, the required surface cleanliness may also be achieved by an inert or reducing gas atmosphere which surrounds the section during the heating phase. This alternative technique falls within the scope of the invention.

In the heating operation, the continuously moving steel section is heated to a temperature close to that of the zinc/zinc alloy coating bath, while preserving the chemical cleanliness of the steel surface sufficient to secure a uniform and tightly adherent coating of the coating medium.

After heating, the chemically clean steel section 16 is passed continuously into a coating device 19 (Figs. 1 and 2) where the section 16 is contacted with zinc/zinc alloy coating liquid. The coating device 19 incorporates a reservoir 20 for heating and maintaining a quantity of the coating material 21; a pump 22 or other means to convey or lift the coating material 21; an insulated trough 23 (preferably insulated with an inner ceramic material lining) , tube or other receptacle in which the coating material 21 is contacted with the moving steel section 16. The insulation of the trough maintains the coating metal at the desired temperature and minimises coating metal loss through reaction of the turbulent coating metal with the trough walls. A coating control device in the form of an air knife (or knives) or wiper gas ring 24 is used to remove excess coating material from the steel section 16 prior to exiting the coating device 19.

An important feature of the coating device is the angle of impingement of the air or gas stream upon the surface of the coated hollow section. To achieve the desired coating thickness, it is necessary to ensure that this angle of impingement is 45° or greater relative to the section surface. While too great an angle (i.e. close to perpendicular) is undesirable due to the necessity of cleaning away the excess liquid coating metal, impingement angles of between 70 and 85° to the section are preferred, however angles down to 45° are acceptable. Use of impingement angles between 70 and 85° allow for coating thicknesses to be less than 150g/m and as low as 2 80g/m whilst maintaining a workable clearance between the air knives and the steel section 17. This is a marked improvement over other conventional means which are in the

2 2 range of 180g/m to 250g/m .

Below is a table showing the average thickness of zinc coating for each of three tube sizes at two different impingement angles taken over a number of runs for each angle.

The coating device 19 may also incorporate facilities to shroud the various surfaces of the coating material 21 and the steel section 16 under an inert or reducing gas atmosphere, either wholly or in part by means of hood 31 and ducts 32.

In the preferred embodiment the steel section 17 passes from the heating apparatus 18 into the coating device 19 and passes through the insulated trough 23 which is continuously supplied with the coating material 21, being molten zinc. A system of weirs (not shown in Fig. 2) assists in maintaining a satisfactory level of coating material 21 in the trough 23, excess coating material being returned to the reservoir 20 after cascading through and/or over the weir plates.

The pump 22 continuously conveys coating material 21 from the reservoir 20 to the trough 23. The reservoir 20 is located directly beneath the trough 23, while a protective gas atmosphere is maintained over the complete coating area. A wiper gas ring 24 supplied with inert gas, is used to remove the excess coating material and controls the final coating thickness.

The pump 22 is configured to allow for quick drainage of the coating material 21 from the trough 23 to the reservoir 20 when it is desired to stop the processing line. The pump 22 is also configured to allow for prompt pump changeover.

This is achieved by means of a "sliding joint" fitting (not shown) which attaches the pump to the wall of trough 23, at a height which permits the trough contents to drain back through the pump body. The fast drainage thereby achieved greatly decreases the possibility of the coating metal freezing within the trough 23. Such freezing results in plant downtime and loss in material yield, which is of prime concern in processing heavy gauge sections. The "sliding joint" fitting allows for fast changeover of the pump assembly.

The choice of the coating material 21 is not limited to zinc or zinc alloys. In particular, coating alloys consisting primarily of differing proportions of zinc and aluminium (e.g. Zn-0.2% Al, Zn-5% AL, 45% Zn-55% Al) and zinc and nickel (e.g. Zn-0.1% Ni) are within the capabilities of the process.

Subsequent to coating, the steel section 16 is conveyed through a cooling stage, where the section is cooled in a controlled manner to achieve optimum properties in the coating. Part quenching, slow cooling or temperature maintenance (e.g. by gas or electric heating) can be used to suppress or enhance certain characteristics of the coating and the nature of its bond to the steel substrate. In particular, delayed quenching or a period at elevated temperature followed by controlled cooling could be used to suppress or enhance the growth of zinc-iron alloys which normally arise as an integral part of the coating of steel with liquid zinc.

In the preferred embodiment, the steel section 16 passes through quenching station 25 and is quenched in water at ambient temperature to limit the growth of zinc-iron alloys and achieve a coated product of enhanced brightness.

Following controlled cooling of the coated steel section, secondary sizing and/or forming operations, can be performed, although this is not mandatory.

In the preferred embodiment, the coated steel section, passes continuously through sets of forming rolls 27 to achieve final shapes which can be square, rectangular, or circular in profile. This same operation is also used to ensure compliance of the end product with dimensional tolerances.

Following any final shaping and/or sizing of the coated steel section it may be overcoated with a range of finishes to enhance corrosion resistance, appearance etc. These finishes may include clear, pigmented, organic or inorganic films.

In the preferred embodiment, the coated steel section is rinsed at rinsing station 33 and spray coated with a clear polymer as it passes through coating station 28. The clear polymer coating provides storage/transport protection to the zinc coated surface, and also acts as a preliminary coating suitable for subsequent overpainting by end-users. A system of air knives 29 controls the polymer thickness, while a drying section 26 is provided to ensure satisfactory curing.

In the final step the coated steel section is fed through a suitable device to cut the continuously moving section to a required length, whereupon the product is checked for quality and assembled for packing.

In the preferred embodiment the coated steel section is continuously fed through and cut by a flying saw 30 prior to being unloaded at unloading station 34.

The present invention therefore provides a method for producing galvanised pipe and other heavy structural galvanised sections in a continuous process.

It should be obvious to persons skilled in the art that numerous variations and modifications could be made to the method and apparatus of the present invention as described and with reference to the drawings without departing from the overall scope or spirit of the inventio .

Claims

THE CLAIMS: 1. A method of producing a hot dip metal coated elongate structural element from a continuous steel strip comprising the steps of:

(a) shot blasting the steel strip;

(b) forming the thus shot blasted steel strip into a structural element of a desired cross section;

(c) pickling said formed structural element;

(d) heating said structural element to a suitable temperature range;

(g) passing said structural element through a cooling and/or quenching station.

2. A method of producing a hot dip metal coated elongate structural element as claimed in claim 1, wherein said liquid metal bath comprises a reservoir for storing and heating liquid-metal, a means of lifting said liquid metal to a receptacle insulated with a ceramic inner lining through which said structural element passes.

3. A method of producing a hot dip metal coated elongate structural element as claimed in claims 1 or 2 wherein subsequent to step (c) and before step (d) the structural element is further treated by rinsing or rinsing followed by steam blowing or rinsing followed by the application of a flux coating.

4. A method of producing a hot dip metal coated elongate structural element as claimed in claim 3 wherein said flux coating is a solution of zinc chloride and ammonium chloride.

5. A method of producing a hot dip metal coated elongate structural element as claimed in claims 1 or 2 wherein during the heating phase of step (d) an inert or reducing gas atmosphere is applied to the structural element.

6. A method of producing a hot dip metal coated elongate structural element as claimed in any one of the preceding claims wherein said coating control device of step (f) is an air knife or gas wiper ring impinging at least one stream of air or gas upon the surface of the structural element at an angle in the range of 45^° to less than normal against the travel of the structural element.

7. A method of producing a hot dip metal coated elongate structural element as claimed in claim 6 wherein said angle is in the range of 70^° to 85°.

8. A method of producing a hot dip metal coated elongate structural element as claimed in any one of the preceding claims wherein said liquid-metal is a zinc or zinc alloy.

9. A method of producing a galvanised elongate hollow structural section from steel strip in a continuous process comprising the steps of:

(a) shotblasting the steel strip;

(b) forming the thus shot blasted steel strip into a substantially circular cross section with an open seam;

(c) passing the substantially circular cross section through a welding station and welding of the open seam thereby forming a hollow section;

(d) pickling the thus formed hollow section;

(f) heating said hollow section to a desired temperature range;

(h) passing the thus galvanised hollow section through a coating thickness control device; and

(i) passing the galvanised hollow section through a quenching and/or cooling station.

10. A method of producing a galvanised elongate hollow structural section as claimed in claim 9 wherein during the heating phase of step (f) an inert or reducing gas atmosphere is applied to the structural section and step (e) is not performed.

11. A method of producing a galvanised elongate hollow structural section as claimed in claim 8 wherein said coating thickness control device of step (h) is an air knife or gas wiper ring impinging a stream of air or gas upon the surface of the structural element at an angle in the range of 45° to less than normal against the travel of the structural element.

12. A method of producing a galvanised elongate hollow structural section as claimed in claim 9 wherein said angle is in the range of 70° to 85°.

13. A method of producing a hot dip metal coated structural section as herein described and with reference to the accompanying drawings.