GB1594351A - Rolling of steel strip - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO THE ROLLING OF STEEL STRIP (71) We, G.S.K. STEEL DEVELOP MENTS LIMITED, a British company of Bank Buildings, High Street, Chepstow, Gwent NP6 5XQ, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the rolling of steel strip and more particularly to the hot rolling of steel slab to strip.

There is a technological problem in designing a wide hot strip mill which is capable of economic operation at medium tonnage rates of less than about one million tons per annum.

In its present form the conventional continuous hot strip mill requires the use of 5 or 6 roughing stands, followed by a 6 or 7-stand finishing train to achieve the overall reduction in thickness from a slab. usually in the range of 200 to 250 mm thickness, down to hot-rolled coil of gauge 2 mm to 3 mm. A mill of this type is characterised by high capital cost and a high intrinsic rolling capacity. An annual tonnage output of about 3 million tonnes of hot rolled coil, for example. would be typical for a modern fully continuous mill of about 1.5 metre width. An alternative mill configuration is available in the form of the serni-continuous mill, comprising a reversing roughing mill (usually making 5 passes) followed by a continuous multi-stand finishing train. The semi-continuous mill is of shorter length and lower capital cost than the fully continuous type but it is still a high tonnage-capacity unit.

The requirement of a high coil weight for the finished product tends to increase the overall length of the mill and necessitates the use of higher rolling speeds to avoid excessive temperature loss of the steel in its passage through the mill. The temperature of the steel at the last finishing stand, and the problem of tail-end temperature loss. is one of the most critical factors determining the operating speed of the mill.

It is an object of the present invention to provide a strip mill of more compact length than has hitherto been available and which capable of rolling at lower speeds than are conventionally used at present, and in the process to allow economic production of wide (wider than about 73 metre) hot-rolled coil at a rate of some half a million to one million tons per annum.

The underlying concept upon which this invention is based is the introduction of an intermediate in-line thermal shield or furnace between the roughing mill and the finishing train of a hot strip mill. This thermal shield serves the purpose of minimizing further heat loss from the slab after completion or rolling through the roughing mill and during the time when the slab is being fed into the finishing train. This enables lower rolling speeds and lower driving power to be employed in the finishing train. It also enables the temperature of the steel to be controlled accurately at entry to the finishing train and a substantially uniform temperature to be maintained along the length of the strip at each finishing stand. A further advantage of this method of temperature control is that it enables a constant entry speed to be maintained at the finishing train.

Thus in one aspect this invention consists in a continuous or semi-continuous strip-mill for the hot rolling of steel slab into strip, comprising; (a) a roughing mill for partial reduction of the steel slab, (b) an in-line roller-hearth tunnel furnace with heated walls into which the partially reduced slab is passed and in which it remains in flat and uncoiled form, and (c) a multi-stand finishing mill-train adapted to reduce the partially-reduced slab to produce strip of the required thickness.

The tunnel-furnace may be heated either by a multi-zone gas or oil-fired burner system, or alternatively by means of electric resistance heating elements.

Preferably a non-oxidizing atmosphere is maintained within the tunnel-furnace to prevent secondary scale formation.

Preferably the temperature of the partially-reduced slab leaving the tunnel furnace is adjusted to a precise figure between 1000 and I l00'C. This is to provide a temperature of the strip leaving the last finishing stand of between x5t) and 90()"C.

The finishing train of the strip mill preferably comprises a number of closely-spaced rolling mill stands (suitably between 4 and 6).

Preferably the finishing train is operated at constant entry speed by virtue of the tact that the slab temperature is controlled independantly by the tunnel-furnace.

Preferably one stand of the finishing train (described as the reference stand) is driven at constant speed by an alternating-current synchronous motor.

Preferably one or more stands are operated with a pre-determined fixed reduction irrespective of the finished gauge of the strip being rolled.

The invention will be further described by reference to the following specific embodiment which describes a strip-mill according to this invention. The dimensions. temperatures and speeds given are by way of example only.

Steel slabs, typically of 200 mm thickness.

1 to 1.5 metres width and about 10 metres length are initially heated to about 1200"C in a slab-heating furnace of known design.

They are then passed through a roughing mill which comprises a descaling unit and reversing rougher of conventional design.

The slab thickness is reduced to about 36 mm by means of five passes through this reversing rougher. The slab then enters a gas-fired tunnel furnace with a temperature ranging from 1075"C at the front end of the slab to 1040"C at the rear end. In the absence of this furnace the slab temperature would fall at a rate of about 2"C per second due to thermal radiation. The inner surface of the tunnelfurnace is maintained at about 1100 C. The exit temperature of the slab, as it leaves the tunnel furnace, prior to entering the finishing train is uniformly within the range 1075on and 1080"C over its whole length. The slab then enters a finishing train comprising six consecutive rolling mill stands, the first of which operates at constant speed and with a fixed reduction ratio. In the first stand the thickness of the slab is reduced from 36mm to about 18 mm and this is further reduced during passage through the other five stands to a minimum value of about 2 mm. The linear speed of the slab entering the finishing train is about 0.4 metres per second and leaving the finishing train is about 8 metres/ second. The strip leaving the finishing train is cooled by water sprays in the normal way as it passes along the run-out table in order to reach the downcoiler at the correct temperature for coiling. Because of the relatively slow speed of the finishing train, however, the run-out table length is significantly shorter than would be necessary in the case of a conventional strip mill.

WHAT WE CLAIM IS:- i. A continuous or semi-continuous strip-mill for the hot rolling of steel slab into strip. comprising: (a) a roughing mill for partial reduction of the steel slab, (b) an in-line roller-hearth tunnel furnace with heated walls into which the partially-reduced slab is passed and in which it remains in flat and uncoiled form, and (c) a multi-stand finishing mill-train adapted to reduce the partially-reduced slab to produce strip of the required thickness.

2. A strip-mill as claimed in claim 1 in which the tunnel-furnace walls are heated by electrical-resistance heating.

3. A strip-mill as claimed in claim I in which the tunnel-furnace walls are heated by burning gas or oil.

4. A strip-mill as claimed in claims 1, 2 or 3 in which an inert or non-oxidising atmosphere is maintained within the tunnelfurnace.

5. A strip-mill as claimed in claim I in which one stand of the finishing mill-train is driven at a constant speed and with a predetermined fixed reduction.

6. A continuous or semi-continuous strip-mill for the hot rolling of steel slab into strip substantially as hereinbefore described.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. of the strip leaving the last finishing stand of between x5t) and 90()"C. The finishing train of the strip mill preferably comprises a number of closely-spaced rolling mill stands (suitably between 4 and 6). Preferably the finishing train is operated at constant entry speed by virtue of the tact that the slab temperature is controlled independantly by the tunnel-furnace. Preferably one stand of the finishing train (described as the reference stand) is driven at constant speed by an alternating-current synchronous motor. Preferably one or more stands are operated with a pre-determined fixed reduction irrespective of the finished gauge of the strip being rolled. The invention will be further described by reference to the following specific embodiment which describes a strip-mill according to this invention. The dimensions. temperatures and speeds given are by way of example only. Steel slabs, typically of 200 mm thickness.

1 to 1.5 metres width and about 10 metres length are initially heated to about 1200"C in a slab-heating furnace of known design.

They are then passed through a roughing mill which comprises a descaling unit and reversing rougher of conventional design.

The slab thickness is reduced to about 36 mm by means of five passes through this reversing rougher. The slab then enters a gas-fired tunnel furnace with a temperature ranging from 1075"C at the front end of the slab to 1040"C at the rear end. In the absence of this furnace the slab temperature would fall at a rate of about 2"C per second due to thermal radiation. The inner surface of the tunnelfurnace is maintained at about 1100 C. The exit temperature of the slab, as it leaves the tunnel furnace, prior to entering the finishing train is uniformly within the range 1075on and 1080"C over its whole length. The slab then enters a finishing train comprising six consecutive rolling mill stands, the first of which operates at constant speed and with a fixed reduction ratio. In the first stand the thickness of the slab is reduced from 36mm to about 18 mm and this is further reduced during passage through the other five stands to a minimum value of about 2 mm. The linear speed of the slab entering the finishing train is about 0.4 metres per second and leaving the finishing train is about 8 metres/ second. The strip leaving the finishing train is cooled by water sprays in the normal way as it passes along the run-out table in order to reach the downcoiler at the correct temperature for coiling. Because of the relatively slow speed of the finishing train, however, the run-out table length is significantly shorter than would be necessary in the case of a conventional strip mill.

WHAT WE CLAIM IS:- i. A continuous or semi-continuous strip-mill for the hot rolling of steel slab into strip. comprising: (a) a roughing mill for partial reduction of the steel slab, (b) an in-line roller-hearth tunnel furnace with heated walls into which the partially-reduced slab is passed and in which it remains in flat and uncoiled form, and (c) a multi-stand finishing mill-train adapted to reduce the partially-reduced slab to produce strip of the required thickness.

2. A strip-mill as claimed in claim 1 in which the tunnel-furnace walls are heated by electrical-resistance heating.

3. A strip-mill as claimed in claim I in which the tunnel-furnace walls are heated by burning gas or oil.

4. A strip-mill as claimed in claims 1, 2 or 3 in which an inert or non-oxidising atmosphere is maintained within the tunnelfurnace.

5. A strip-mill as claimed in claim I in which one stand of the finishing mill-train is driven at a constant speed and with a predetermined fixed reduction.

6. A continuous or semi-continuous strip-mill for the hot rolling of steel slab into strip substantially as hereinbefore described.