WO1999061192A1

WO1999061192A1 - Surface treatment of hot metal articles

Info

Publication number: WO1999061192A1
Application number: PCT/GB1998/001519
Authority: WO
Inventors: Michel Detronde
Original assignee: Heckett Multiserv France S.A.
Priority date: 1998-05-26
Filing date: 1998-05-26
Publication date: 1999-12-02
Also published as: AU7542998A

Abstract

A cast article is subjected to scarfing using a torch (1) having a plurality of outlets to produce e.g. a generally parabolic flame to provide a pool of molten metal which is shallow and wide, the outlets from which to supply burning gas being selected so as to scarf the surface and leave a generally flat defect-free surface.

Description

SURFACE TREATMENT OF HOT METAL ARTICLES

The invention relates to the surface treatment of hot metal articles, and in particular to the

treatment of slabs produced by the continuous casting of steel.

It is well known that defects are present in the surface and/or beneath the surface of cast

metal articles. The traditional way of removing the defects is so-called "scarfing" by which the metal area containing the defects is burned away and the defect-free metal areas

are welded together. Scarfing is carried out using a torch having a supply of combustible gases. The front face of the torch has one or more outlets through which the burning

gases emerge, and the tool is oriented so that these burning gases are aimed at the area to

be treated. In some cases the article is allowed to cool and the scarfing is done then by an operator who holds the torch and can approach the relatively cooler slab sufficiently close

to locate and treat the defects.

Slabs or like articles produced by the continuous casting of steel also need a scarfing

treatment. While these can be scarfed in the traditional way the need to allow the slab to

cool down disrupts the continuous fast production processes. The requirements in terms of surface regularity of slabs are very stringent when the slab is to be rolled into thin sheet

for use in say the automotive industry. If the sheet has a slight defect caused by

inadequate scarfing the automobile maker will reject the product. So in addition to the

need to make the scarfing process quicker it must also be more accurate. It has now been realised that for a hot semi-finished metal product, typically above about

450°C, the scarfing should be performed in a specific way

It is one object of this invention to provide a method and apparatus for scarfing metal

slabs which is so fast and reliable that it can be applied to hot metal slabs produced by

continuous casting without disrupting the overall process and which after rolling can

provide defect-free thin sheet metal

According to the invention in one aspect there is provided a method of scarfing a metal

article which is at a temperature above about 450°C, the method comprising exposing a surface of the slab to a gas torch producing a flame to burn away the defects, the gas

torch having a plurality of outlets, the method comprising (i) selecting outlets of the torch from which to supply burning gas so as to scarf the surface and leave a generally flat

defect-free surface, and (ii) passing the article to a reheat step

In another aspect the invention comprises a torch which comprises a generally tubular

body having gas passageways having inlets at a rear end and gas outlets at a front end and

means for controlling the flow of gas from the rear to the front on a selective basis

Preferably the outlets are arranged in a generally circular disposition

Preferably there is an outer zone of gas outlets at the front end, e g for a heating gas

There is a main zone of gas outlets for oxygen, and these may be arranged in sectors Preferably many oxygen outlets are present arranged for use in different combinations according to need. It is preferred that the passageways have individual valves or throttle devices to selectively open or close passageways.

By selecting the individual passageways it is possible to arrange that the torch will have a specific oxygen output speed distribution independent of its cross-section. Such a torch

will:

provide a variable power multi-nozzle torch by adjusting the power of a coherent distribution which does not generate any turbulence, and

provide a bath of molten metal on the surface of the article being scarfed which is

both shallow and wide. This is useful for continuously cast slabs because defects are generally small and located under the surface or skin at a depth ranging

between 1 and 2mm. The width may be 20% wider than the usual pool.

In addition, supplementary nozzles may be present and directed towards the rear edge of the bath, to enable simultaneous trimming and scarfing in an edge zone and on the

surface.

It is a much preferred feature of the invention that the flame has the general shape of a

parabola because that will scarf in a generally flat thin or shallow pattern, and be of an

area substantially equal to that of any area to be scarfed. It will also produce a pool of

liquid metal which is turbulent-free and which may readily be urged away as the slab is moved past or relative to the torch. The pool of molten metal can be driven away as droplets. Often however they are driven to a corner of the slab to solidify and form a fin

or beard. That can be particularly difficult to remove, because it is made up of iron metal instead of iron oxide. The presence of such a fin is unacceptable, especially when thin

sheet is to be rolled. The fins tend to be produced while the slab is moved fast relative to the torch. In such a case the method of the invention is carried out by using gas outlets of

the torch at the appropriate location to burn away the beard.

It will be seen that a gas torch of the invention has the benefit of a jet of the multinozzle

torch and a distribution comparable to that of a single nozzle torch.

It is sometimes necessary to trim a scarfed pass and this is can be carried out using the

same torch immediately following the scarfing pass or after one surface has been scarfed. This action enables the repair to be completed directly on the machine to provide a

product suitable for charging without any other operation, checking, retouching and the

like. For this purpose two or more oxygen nozzles may be placed either side of the torch,

with each nozzle having its own distribution for the fluid chosen (oxygen, oxygen +

heating gas, or high pressure water). In addition the nozzles may be oriented along the

torch directed towards the limit points of the bath. The trimming may be used to remove the fin as it is formed, along the whole pass on slabs positioned at any disposition, e.g.

with the surface horizontal, vertical or inclined. The same or another supplementary

oxygen nozzle may be used to remove any lower fin. The oxygen jet may burn the fin

formed by coagulated metal coming from the melting metal bath. The advantage in using

this nozzle is that the bottom of the pass is well away of any equipment which facilitates the positioning of the torch for the following pass. Moreover, the torch scarfs more

quickly because there is no need to reheat the fin in order to remove it during the following pass. In such a case, scarfing and removal of the fin are carried out

simultaneously, which offers a significant gain in productivity.

The torch is very versatile. For a given form of torch (the example given is a tubular

torch with a round face), the change in the concentric distribution, from, for example, three sectors to four or more sectors, is quickly made by switching within a distribution

cassette inside the torch.

The method includes the further step of shaping the scarfed slab into a coil of thin sheet

metal by rolling.

A torch of the invention may be used to scarf ingots of varying dimensions, blooms, steel

frame heavy profile rough rolled ingots, rails, and the like.

In order that the invention may be well understood it will now be described by way of

example only with reference to the accompanying diagrammatic drawings, in which:

Figure 1 is a stylised front elevation of a scarfing torch, showing the sectors;

Figure 2 is a front elevation of the torch; Figures 3 A and 3B are views showing the scarfing flame pattern under different conditions, and

Figure 4 is a longitudinal sectional view of the torch with the front face to the left

The torch comprises a general tubular body 1 having a main passageway housing a number of pipes P, some supplying heating gas to nozzle outlets at the front face F of the torch and others supplying oxygen to a large number of outlets making up the main area of the front face Each outlet is individually controlled, as will be explained later, but groups of the outlets may be controlled by a central control device or cassette to operate together As shown in the stylised diagram of Figure 1, there is a main core area A, and an outer circle C, between these is a ring B which is divided into two side segments B1,B2 and top and bottom segments BO The flame from the nozzles of each segment provides a portion of the overall flame which has the general the shape of a parabola (the same reference letters are used) Such a flame can make a shallow scarf even at high speed with little or no turbulence In the example shown in Figure 3 A, the segment of the outlets may be operated to scarf a shallow cut, i e across the thin top surface of an entire slab to be scarfed and to produce an ideal parabolic shaped flame The areas between the flames from zones B and C can produce turbulence which should be avoided by adopting the pattern shown in Fig 3B The uppermost layer of the slab is heated to melting point and the slab and the torch are moved relative to each other at high speeds so that the

liquid metal is burned away or blown off The torch of Figures 1 to 3 scarfs and trims slabs both at ambient temperature and at temperatures up to 1000°C . It treats slabs produced by continuous casting. The scarfing

operation forms part of the process of treatment of hot slabs in the production cycle. It enables very shallow passes to be made (approx. 1mm on both slabs at 1000°c and cold slabs). This results in a gain in metal because the minimum is burned away. The torch

produces a wide bath (500mm and more), at a constant displacement speed, it contributes to ensuring the dimensional regularity of the cross section of the slab. This facilitates the bar hold at rolling. There is no sonic wave; because there are many nozzles the pool is

smooth and free of turbulence.

The torch can be fitted with two trimming devices (not shown) which enable a finished slab suitable for charging to be delivered in minimum time which is suited to an increased

production rate. The scarfing machine fitted with a torch of this type is integrated in the equipment chain enabling hot charging of slabs which require repair in the same sequence

as good slabs which do not require scarfing. This results in a significant financial gain for

the iron and steel metallurgist in terms of unused slab stocks and thermal energy saved.

The torch features a tubular body housing a number of pipes which have different

functions. A heating ring 2 for a gas oxygen flame (natural gas, propane, etc.)

dimensioned for scarfing both cold slabs and hot slabs at temperatures up to

approximately 1000°C; and a multi-nozzle oxygen flush nozzle (for example, 100 nozzles

with a hole of 8 mm to 12mm); the blow torch can be disposed at an angle of about 30 to

about 60 degrees with respect to the slab and with respect to its own displacement in order to generate only one burr when passing over the slab positioned with the surface

horizontal, vertical or inclined.

The combustion of the bath is primed by injection of powder or by another system.

Different gases may be circulated to control release of dust and to suppress dust oxidation.

The displacement speed of the blow torch on the slab varies according to the temperature of the slab and can range from approximately 20m/min when cold up to 80 to 90m/min at

1000°C.

The blow torch is capable of working at low gas and oxygen pressures (< 6 bars for heating gas and < 10 bars for flushing oxygen according to the adjustment requirements

of the different concentric zones).

The circular shape used in the example to present the principle is the easiest to build.

However, it is not the only possibility that is interesting. For example the torch may be an

elliptical shaped multi-nozzle jet (main axis parallel to the slab) surrounded by a rectangular "heating ring" in order to improve the pass profile by producing a flatter pass.

By virtue of the shape and arrangement of the outlets one can achieve a high power

output while retaining accuracy of treatment and low gas consumption. One can trim

and/or remove fins, and treat the total surface area of the slab in one step at a speed which

does not disrupt the overall steel processing operation.

Claims

1. A method of scarfing a metal article at a temperature above about 450┬░C, the

method comprising exposing a surface of the slab to a gas torch having a plurality of outlets and producing a flame, the method comprising (i) selecting outlets of

the torch from which to supply burning gas so as to scarf the surface and leave a

generally flat defect-free surface and (ii) passing the article to a reheat step.

2. A method according to Claim 1, wherein the outlets are selected so that the flame

is generally parabolic in shape.

3. A method according to Claim 1 or 2, wherein the flame is adapted to provide a

bath of molten metal which is shallow and wide.

4. A method according to Claim 3, wherein the bath is between 1 and 2mm deep.

5. A method according to Claim 3 or 4, wherein the bath is about 500 mm wide.

6. A method according to any preceding Claim, wherein the flame is shaped to burn

away a fin or beard.

7. A method according to any preceding Claim, wherein the article is 450┬░ to 1000┬░C.

8. A method according to any preceding Claim, wherein the hot metal article is a hot

metal slab, ingot, bloom, or the like.

9. A torch which comprises a generally tubular body having gas passageways having

inlets at a rear end and gas outlets at a front end and means for controlling the flow of gas from the rear to the front on a selective basis.

10. A torch for scarfing a metal article, the torch comprising an elongate body having an inlet at its rear and containing pipes leading to a plurality of nozzles at the

forward end through which burning gas flame emerges, the nozzles being arranged

in groups, one group being arranged to provide a main flame to form a pool of molten metal on the surface to be scarfed, other groups of nozzles being arranged

to provide supplementary flame to adjust the shape of the overall flame to make a

wide shallow cut scarf.

11. A torch according to Claim 9 or 10, wherein the torch is arranged to provide a

flame of generally parabolic shape.

12. A torch according to Claim 9,10 or 11, wherein the nozzles are arranged that the

flames have laminar flow gas.

13. A torch according to any of Claims 9 to 12, wherein the nozzles are arranged in concentric rings.

14. A torch according to any of Claims 9 to 13, wherein the supply pipes for the other groups include valve or throttle means to control the rate of flow of gas.

15. A torch according to Claim 14, wherein the valve or throttle means comprise a diaphragm valve.