WO1995026837A1

WO1995026837A1 - Method of reducing the effects of thermal shock on the rolls of hot reduction mills

Info

Publication number: WO1995026837A1
Application number: PCT/AU1995/000181
Authority: WO
Inventors: Leslie George Gore; Wai Yee Daniel Yuen
Original assignee: Bhp Steel (Jla) Pty. Ltd.
Priority date: 1994-03-31
Filing date: 1995-03-30
Publication date: 1995-10-12
Also published as: AUPM482994A0

Abstract

A method of reducing thermal shock on a work roll during the hot reduction of steel slab or sheet wherein a surface of the workpiece is cooled by at least 150 °C to a temperature of not less than 750 °C prior to its contact with the roll by a spray of liquid coolant. The spray comprises high speed droplets directed towards the workpiece surface, and is positioned as closely as conveniently possible to the roll, consistent with no liquid coolant remaining on the workpiece surface when it contacts the roll.

Description

METHOD OF REDUCING THE EFFECTS OF THERMAL SHOCK ON THE ROLLS OF HOT REDUCTION MILLS TECHNICAL FIELD

This invention relates to the hot reduction of steel slab, plate and strip (all of which are encompassed by the term "workpiece" hereinafter) in rolling mills. That is to say the process of reducing the thickness of a workpiece by passing it while hot between the pressure or work rolls of a roll stand in such a mill.

BACKGROUND ART

Typically, steel slab is heated in a soaking furnace to about

1200°C and passed through one or more roughing roll stands whereby its thickness is reduced substantially before passing directly to a multi- stand, uni-directional finishing line whereby it is progressively reduced to a required finished strip thickness. The slab may enter the roughing stand or stands at a temperature of about 1200°C and the roughed down slab may enter the first stand of the finishing line at about 1000°C. It emerges as strip from the last stand at a temperature that is required to be above the transition temperature of about 750°C, that is to say the temperature at which the internal structure of the steel is likely to change from the austenite phase to the ferrite/pearlite phase.

The cylindrical surface and sub-surface layer of each work roll is heated by contact with the hot workpiece, and conventionally each work roll is cooled by water sprays directed at the exposed surface of the roll. Usually the cooling sprays are directed at a downstream zone of the surface of the roll adjacent to the workpiece where it emerges from between the pair of work rolls. This is because the spray is most effective if it contacts the hottest part of the roll surface. However it is also well known to direct further cooling sprays at an upstream zone of the roll surface before that surface first contacts the workpiece as the workpiece enters the nip of the rolls. Usually wiper means are provided to ensure that the cooling water is kept away from the workpiece, in view of the desirability of keeping the workpiece temperature high and at least above the transition temperature until rolling is completed.

It is also known to spray liquid lubricant into the nip of the work rolls for simultaneous application to the surfaces of the roll and the strip, and such lubricants necessarily have some cooling effect. Such lubricant application is usually associated with cold reduction rolling, but there have been proposals to apply it to hot reduction processes.

The most relevant publications of background art proposals known to applicant are:

Japanese patent application 58-125968 (Nippon Kokan) - Lubricating Method in Cold Rolling, Japanese patent application 55-187562 (Kawasaki Seitetsu) -

Hot Rolling Method,

Japanese patent application 4-197097 (Sumitomo Light Metal Industries) - Device for Removing Coolant of Rolling Mill, Japanese patent application 56-10593 (Kobe Seikosho) - Rolling Method for Plane Shape Controlling, and

Russian (former Soviet Union) patent application 1761322 (Dnepr Metal Inst.) - Lubrication and Cooling of Hot Rolling Mill Rolls.

Consistent with established prior art, those publications disclose the application of lubricant or coolant either to the roll surface alone, or, by virtue of steeply inclined nozzles directed towards the line of initial contact of the workpiece and the roll, simultaneously to the roll surface and the workpiece surface. Perhaps more significantly, wherever cooling is the primary purpose, direct cooling of the roll by application of the coolant to the roll surface is envisaged.

DISCLOSURE OF INVENTION

When a roll is in use, a short circumferentially extending zone of its cylindrical surface is rapidly heated by contact with the workpiece and is then cooled as it moves around the roil until it once more contacts the workpiece. With respect to the roll, once a stable operating condition has been reached, the heat input from the workpiece and the heat loss to the coolant during each revolution are of approximately equal magnitude, and the interior core of the roll maintains a substantially constant temperature.

Typically, the core temperature is less than 100°C, say from 60°C to 80°C, whereas a thin surface layer of the contact zone is heated rapidly to about 550°C by contact with the workpiece, then cooled to something less than the core temperature and then raised to near the core temperature, by conduction from the interior of the roll, before being again heated. This high speed repetitive temperature cycle produces so called thermal shock which is deleterious to a surface layer of the roll.

Specifically, fatigue cracks, known as "fire cracks" propagate from the surface into the sub-surface of the roll. Those cracks eventually lead to spalling and so called peeling of the surface layer, which requires the roll to be taken out of service and re-surfaced, by grinding or otherwise, or replaced. From investigations and analyses leading to the present invention it appears that fire cracks propagate to a depth that depends on the maximum temperature reached by the material of the roll, being limited to material that reaches a temperature above about 300°C during the thermal cycle. Under normal practice this equates to a depth of about 1 mm from the surface of the roll.

The maximum temperature reached by the roll surface is theoretically, and in practice very close to, the mean of the core temperature and the workpiece temperature immediately prior to contact with the roll, that is, as indicated above, a temperature of about 550°C under conventional practice.

An object of the present invention is to reduce that maximum temperature, thereby reduce the depth of roll prone to fire cracking, and so increase the working life of the roll between re-surfacing operations.

The invention achieves that object by reducing the surface temperature of the workpiece immediately prior to it contacting the roll, without substantially reducing the heat content of the workpiece as a whole.

According to one aspect the invention consists in a method of reducing a hot steel workpiece by passing the workpiece between a pair of work rolls, wherein at least one surface of the workpiece is cooled, without appreciably reducing the temperature of the body of the workpiece, prior to its contact with one of said rolls by a spray of liquid coolant directed so that substantially all of the coolant strikes directly against the workpiece surface, whereby the effect of thermal shock on said one roll is reduced. This may be distinguished in principle from the prior art wherein the objective is to cool and/or lubricate the roll, and the lubricant/coolant is directed onto the roll surface.

According to preferred embodiments of the above described invention it consists in a method of hot reducing a steel workpiece by passing the hot workpiece between two work rolls, wherein; coolant is applied to a stationary zone of a moving surface of the workpiece, being a surface that is contacted by at least one of said rolls, said zone is spaced from a line of first contact between said moving surface and said at least one roll by a distance not exceeding 500 mm, the width of said zone and the speed of movement of said workpiece is such that any point on said moving surface resides in said zone for no more than 0.5 seconds, and the coolant is applied so as to reduce the temperature of the moving surface as it traverses said zone by at least 150°C to a value not less than 750°C.

To enable the rapid pull down of surface temperature with a minimum overall heat loss from the workpiece that characterises preferred embodiments of the invention, the coolant is preferably water and is applied as a spray that comprises small, high speed droplets striking the workpiece surface substantially perpendicularly thereto. This enables the individual droplets to penetrate the steam generated by earlier applied coolant and break through the thermal barrier, that is to say come into immediate good heat transfer relationship with the workpiece surface. This, in turn, ensures that a minimum of water may be used (ideally all of the water is flashed to steam), and produces optimal conditions for maximising the temperature reduction of the surface for a given overall heat extraction from the workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example, an embodiment of the above described invention is described in more detail hereinafter with reference to the accompanying single figure drawing, being a diagrammatic end elevation of a pair of work rolls in combination with workpiece surface cooling means whereby the method of the invention may be effected.

BEST MODE OF CARRYING OUT THE INVENTION

In the drawing there is disclosed a workpiece slab 1 passing between two work rolls 3 and 4 respectively, and a spray head 5 directing a coolant spray 6 onto the lower face of the workpiece 1. The drawing is purely diagrammatic and is not to scale. In particular the rolls 3 and 4 are drawn undersized, so that the fraction of the roll circumference shown in contact with the workpiece is considerably more than it would be in actuality.

As a rule, the lower face of the workpiece is hotter than the upper face because of the latter's proximity to the floor and the table rolls, which tend to reflect radiated heat back to the workpiece. Thus the lower roll of each stand is apt to suffer more from thermal shock than is the upper one, and in some instances only the lower surface of the workpiece may be cooled in accordance with the invention. However it will be appreciated that in other instances both faces of the workpiece are cooled. The spray head 5 emits a plurality of closely spaced jets 6 of small high speed droplets, and it will be apparent from the drawing that these impinge directly upon the workpiece 1. Moreover the droplets ideally travel along paths that are substantially perpendicular to the surface of the workpiece, while taking into account the desirability of avoiding coolant falling back onto the nozzles when cooling the underside of the workpiece.

The "footprint" of the spray 6 upon the workpiece surface, that is to say the stationary zone of the workpiece surface struck by the spray, has a length equal to the width of the workpiece, so that the entirety of the lower surface of the workpiece is wetted as it travels past the spray head 5, and may have a width in the direction of workpiece travel of from 5 to 200 mm, preferably of the order of 10 mm. The footprint is spaced from a line of first contact 7 between the workpiece 1 and the roll 4 by a maximum distance of 500 mm. Preferably however the spray footprint is as close as conveniently possible to the line of first contact 7, consistent with little or no liquid coolant remaining on the workpiece 1 when it reaches the roll 4. This requirement typically requires the relevant distance (dimension D in the drawing) to be within the range of from 150 mm to said maximum, preferably about 350 mm.

For preference the coolant is water at ambient temperature and it may be applied to a typical workpiece of say 2 to 2.5 metres width travelling at from 30 to 300 metres per minute at a rate of from 500 to 3000, preferably 1500, litres per minute. This equates to a preferred application rate of from 4 to 25 litres, preferably 12.5 litres, of water per metre of footprint length per second. For a footprint of 10 mm width in the direction of workpiece travel, this, in turn, equates to a "spray flux density" of 1250 litres per square metre of footprint area per second, with proportionately greater or lesser spray flux densities for narrower or wider footprints respectively.

To achieve that spray flux density and appropriate droplet size and speed, the spray head 5 may provide a line of nozzle outlets, for example from 4 to 8, preferably 6, equally spaced apart outlets per metre of spray head length, if supplied from a water supply sufficient to maintain a pressure within the spray head 5 of the order of 600- 700 kPa.

In a typical hot mill it has been found, using a 10 mm wide footprint, spaced 150 mm from the line of first contact with a spray flux density od 1060 at a pressure of 600 kPa and with the nozzles 100 mm vertically from the lower surface of the workpiece, that the lower work roll in the first finishing stand exhibits a decrease in roll peel of at least 50%, while the lower work roll in the second finishing stand exhibits of a decrease of at least 20%

Claims

1. A method of reducing a hot steel workpiece by passing the workpiece between a pair of work rolls, wherein at least one surface of the workpiece is cooled, without appreciably reducing the temperature of the body of the workpiece, prior to its contact with one of said rolls by a spray of liquid coolant directed so that substantially all of the coolant strikes directly against the workpiece surface, whereby the effect of thermal shock on said one roll is reduced.

2. A method according to claim 1 wherein the temperature of the workpiece surface is reduced by at least 150°C to a value not less than 750°C.

3. A method according to claim 2 wherein the coolant spray comprises droplets directed towards the workpiece surface along paths that are substantially perpendicular to the workpiece surface.

4. A method according to claim 2 wherein said coolant strikes a stationary zone of said workpiece surface having a width in the direction of workpiece travel of from 5 to 200 mm.

5. A method according to claim 2 wherein said coolant spray strikes a stationary zone of said workpiece surface that is spaced from a line of first contact of the workpiece surface with said one roll by a distance within the range of from 150 to 500 mm.

6. A method according to claim 5 wherein said stationary zone is positioned as closely as conveniently possible to said one roll consistent with no liquid coolant remaining on the workpiece surface when it contacts said one roll.

7. A method according to claim 5 wherein said zone is spaced from a line of first contact between the workpiece surface and said one roll by a distance of not more than 350 mm.

8. A method according to claim 2 wherein said coolant is water at ambient temperature and said spray head applies coolant to said zone at a rate of from 500 to 3000 litres per metre of zone length per second.

9. A method according to claim 7 wherein said spray is delivered from a spray head providing a line of from 4 to 8 equally spaced apart outlets per metre, supplied from a water supply maintaining a pressure within the spray head of from 600 to 700 kPa.

10. A method of hot reducing a steel workpiece by passing the hot workpiece between two work rolls, wherein; coolant is applied to a stationary zone of a moving surface of the workpiece, being a surface that is contacted by at least one of said rolls, said zone is spaced from a line of first contact between said moving surface and said at least one roll by a distance not exceeding 500 mm, the speed of movement of said workpiece is such that any point on said moving surface resides in said zone for no more than 0.5 seconds, and the coolant is applied so as to reduce the temperature of the moving surface as it traverses said zone by at least 150°C to a value not less than 750°C.