WO1997001403A1

WO1997001403A1 - Method and plant for the manufacture of a thin hot-rolled steel strip

Info

Publication number: WO1997001403A1
Application number: PCT/EP1996/002876
Authority: WO
Inventors: Cornelis Pronk; Huibert Willem Den Hartog; Marcus Cornelis Maria Cornelissen
Original assignee: Hoogovens Staal B.V.
Priority date: 1995-06-29
Filing date: 1996-06-28
Publication date: 1997-01-16
Also published as: AU6516696A; ZA965577B; NL1000696C2

Abstract

The manufacture of thin hot-rolled steel strip comprises the steps of: (i) forming a liquid steel into a cast slab having a thickness of less than 100 mm in a continuous casting machine; (ii) rolling the slab in the austenitic region into an intermediate slab having a lesser thickness than the cast slab; and (iii) rolling the intermediate slab in the austenitic region into a strip. The slab or the intermediate slab is held for a period of time in a furnace in which a non-oxidizing atmosphere is maintained in contact with its surface. The thickness of the intermediate slab is in the range 5 to 25 mm and in step (iii) the intermediate slab is rolled to a final thickness of less than 1.2 mm. This enables production of particularly thin hot-rolled strip, using a simple plant.

Description

Title

METHOD AND PLANT FOR THE MANUFACTURE OF A THIN HOT-

ROLLED STEEL STRIP

Field of the invention

This invention relates to a method of manufacture of thin hot-rolled steel strip, and to plant for use in such a method, as well as to components useful in such a plant.

Description of the prior art

WO 89/11363 (EP-A-415987) describes a method for the manufacture of a thin hot-rolled steel strip in which liquid steel is formed in a continuous casting machine into a thin slab with a thickness smaller than 100 mm, and, with reheating in a furnace, the steel slab is rolled in the austenitic range into a desired finished thickness in a number of process stages following one another. A problem with this known method is associated with its continuous character. The continuous character limits the rolling speed to a speed corresponding with the casting speed and leads to the steel being exposed to the air while it is being subjected to high temperature so that an oxide scale forms . The oxide scale cannot be removed or only in very small part by conventional high pressure water sprays . In particular during the later rolling stages in the rolling process a stubborn, tenacious oxide is formed that in practice cannot be removed with water sprayers. Oxide formation combined with the relatively slow rolling speed sets an upper limit to the temperature at which the steel can be rolled in the austenitic range. On the other hand the inevitable cooling during rolling limits the time during which it is possible to roll. In addition in the further rolling the amount of surface per volume unit of heat- retaining steel reduces . All combined these factors lead to the minimum thickness of the hot-rolled steel being limited. In a preferred embodiment, WO 89/11363 cites 4.05 mm.

EP-A-504099 describes a process in which a slab is continuously cast at a thickness, following "squeezing" before the core is solidified, of 45 mm. In a single roll stand, this thickness is reduced to 15 mm. Subsequently this slab may be re-heated, and it may then be coiled. It is thereafter rolled in a continuous rolling, first in the austenitic region to 1.5 mm and then in the ferritic region to 0.7 mm.

In a conventional hot strip installation the rolling speed is not limited by the capacity of a preceding installation, but for the rest the effects described above occur likewise. For this reason in the conventional installations no hot rolled steel is usually made any thinner than 1.8 mm. Under optimum conditions 1.3 mm may be considered as just possible to achieve. A consequence of this is that for many applications in which a hot-rolled strip would be usable on account of requirements relating to shape, surface condition or formability, a thinner cold-rolled steel is still used. Applications which may be mentioned here include steel pipes, steel vessels and building applications such as cladding, roof coverings and sections.

Another important drawback is that the relatively thick hot-rolled strip is less suitable for use in manufacturing deep-drawing steel for making beverage or drinks cans, for example. In order to obtain the desired thin steel for beverage cans the hot-rolled steel must undergo a big reduction in the ferritic range. The big reduction, in particular with low- carbon steel, causes great anisotropy to occur in the cold-rolled steel. As a consequence of the difference in deep-drawing properties in different directions, this anisotropy results in big differences in height around the circumference of a deep-drawn and ironed can. In order to make the height all round uniform again much material needs to be cut away and this leads to material loss. Therefore, the packaging industry has been obliged to fall back on the much more costly "ultra-low-carbon" ULC steel or has resigned itself to a compromise between material thickness and material losses through cutting.

In connection with oxide control or prevention in hot-rolling, it is mentioned that proposals have been made for parts of the hot-rolling plant to have a non¬ oxidizing or low-oxidation atmosphere. See for example JP-A-62-89501, JP-A-1-130802 and JP-A-58-32506.

Summary of the invention

The object of the invention is to provide a method by which it is possible to manufacture a hot-rolled steel strip of a thickness which is smaller than is achievable with the state of the art and which furthermore may be carried out to economic advantage and with simple technology. According to the invention there is provided a method of manufacture of thin hot- rolled steel strip, comprising the steps of

(i) forming a liquid steel into a cast slab having a thickness of less than 100 mm by means of a continuous casting machine, (ii) rolling the slab in the austenitic region into an intermediate slab having a lesser thickness than the cast slab, and (iii) rolling the intermediate slab in the austenitic region into a strip, wherein the slab or the intermediate slab is for a period of time in a furnace in which a non-oxidizing atmosphere is maintained in contact with the surface thereof, and the thickness of the intermediate slab is in the range 5 to 25 mm and in step (iii) the intermediate slab is rolled to a final thickness of less than 1.2 mm. Preferably the thickness of said intermediate slab is in the range 5 to 20 mm.

Maintaining a non-oxidising atmosphere at the surface of the slab prevents oxide formation or at least considerably restricts it. This means that, even at relatively low rolling speeds, a high rolling temperature may be applied which permits hot-rolling to a small finished thickness without reaching the two- phase, austenitic-ferritic range. During hot-rolling the surface in particular of the steel slab cools considerably. In order to reheat the surface it is known to allow the slab to pass through a furnace apparatus, for example an induction furnace. For this a particularly effective embodiment of the invention may be used in that at least the furnace apparatus a non-oxidising or low oxygen gaseous atmosphere is present.

The steel slab stays for a relatively long time in the furnace wherein the surface of the slab may increase considerably in temperature. Applying and maintaining a non-oxidising or low-oxygen atmosphere in the furnace prevents or at least restricts oxide formation.

As stated the surface in particular of a steel slab drops considerably in temperature during rolling and the surface of the steel slab increases considerably in temperature during heating in a furnace apparatus. For homogenizing the temperature, it is known to use a homogenizing apparatus, preferably a coiling apparatus in which the slab is stored temporarily in the form of a coiled coil. In a particularly effective embodiment of the present invention a non-oxidising gaseous atmosphere is maintained in the coiling apparatus. A non-oxidising or low-oxygen gaseous atmosphere in the coiling apparatus prevents or restricts an oxide scale forming on the surface during the relatively long stay of the slab in the coiling apparatus at a relatively high temperature.

Combining the two embodiments of the invention just described achieves the effect that during a considerable part of its production time the steel slab stays in a non-oxidising or low-oxygen atmosphere. This allows only a very small amount of oxide to form or none at all .

The invention makes it possible to prevent or reduce oxide formation. This frees the way towards making a hot-rolled steel strip with a thickness smaller than the thickness of 1.3 mm which may presently be achievable in practice. This steel strip may also be used where at present a much more costly cold-rolled strip is being used. It is also possible to use the thin hot-rolled strip as base material for cold-rolling into formable steel for manufacturing beverage cans or other greatly formed products. Because the material needs to be rolled less in the ferritic range, the anisotropy is limited and the use of low- carbon steel (e.g. 0.01-0.1%C) becomes possible instead of the much more costly ultra-low-carbon steel.

It is of advantage to make the entry speed of the relatively hot strip into the mill stands of the rolling train after the furnace apparatus and/or coiling apparatus as high as possible in order to make as brief as possible the time that the surface of the slab is exposed to the outside air as it enters the mill stands. Therefore it is preferable to feed the slab into the mill stands after the furnace apparatus and/or coiling apparatus with a thickness of between 5 and 20 mm. In order to keep the time of exposure brief it is preferable to select an entry speed higher than 0.6 m/s, preferably higher than 0.8 m/s. With a small thickness the exit speed then remains within practically controllable limits of 800 m/min, preferably around 600 m/min. In order to keep the rolling forces low, to achieve a good surface quality and to introduce the deformation homogeneously over the thickness, it is preferable that lubrication rolling is carried out in at least one mill stand, more preferably the second mill stand. In connection with a stable feed into the first mill stand, the largest reduction is made in the second mill stand. Another aspect of the invention is to provide a plant for the manufacture of a thin hot-rolled steel strip. According to the invention in this aspect, there is provided a plant for the manufacture of thin hot- rolled steel strip, comprising

(a) a continuous casting machine for casting a steel slab,

(b) a furnace apparatus arranged for receiving steel slab cast in the continuous casting machine, for adjusting the temperature of the steel slab, the furnace apparatus having an entry and an exit for the slab and an enclosed path for the slab from the entry to the exit,

(c) a coiling apparatus for receiving the steel slab from the furnace apparatus, coiling the slab and subsequently uncoiling the slab, the coiling apparatus having an enclosure providing an enclosed space in which the slab is coiled,

(d) rolling apparatus for receiving the steel slab uncoiled from the coiling apparatus and rolling the slab into strip of a desired thickness, and

(e) means for providing a non-oxidising gas atmosphere in at least the furnace apparatus at the path thereof wherein the exit of said furnace apparatus is substantially gas-tightly connected to the coiling apparatus . Such an apparatus and its advantages and specific embodiments are described in the International patent application "Plant for the manufacture of steel strip" with the same filing date as the present application and in the name of the same applicant, with reference no. HO 848. The content of that application is deemed to be included in the present application by this reference.

The furnace apparatus and coiling apparatus are preferably detachably coupled to each other. The coiling apparatus may also be provided with a non¬ oxidizing atmosphere. By this plant there is achieved the effect that from the time when the slab runs into the furnace apparatus until the time it is conveyed out of the coiling apparatus, the slab does not come into contact with the outside air, but rather it is continually surounded by a gaseous atmosphere of a non¬ oxidizing composition. For this purpose the gaseous atmosphere in the furnace apparatus and in the coiling apparatus may be the same or different .

The gas atmosphere provided in the furnace apparatus and preferably also in the coiling apparatus is substantially non-oxidizing, though inevitably it may include a small amount of oxygen due to leakage of air. Preferably it is based on nitrogen, although an inert gas such as argon may be used if its high cost allows. The nitrogen may contain additive for inhibiting nitriding of the steel surface, as is known in the process of batch annealing of steel . The gas atmosphere may contain water vapour.

Typically the furnace apparatus is built as an electric furnace in which, by means of resistance or inductive heating, energy is supplied to the slab, so that in any event the surface of the slab is heated again after having cooled as a consequence of the descaling by high pressure water sprays and because of heat loss to the surroundings. In the case of conventional plants, during this heating the surface is exposed to the normal outside atmosphere along a relatively great distance and thus for a relatively long time, so that an oxide scale again forms on the surface, which under these conditions is a thin, tenacious layer which in practice cannot be completely removed with available very high water pressures and which ultimately must be removed by pickling.

The furnace apparatus may be employed only for homogenizing the temperature of the steel slab, or may be arranged to alter at least the core of the slab in temperature.

In the plant in accordance with the invention the slab is prevented from coming into contact with the outside atmosphere as it passes through even a relatively long furnace apparatus, so that oxide scale thereby forming on the outer surface of the slab is minimized.

As stated, the coiling apparatus is provided an enclosure, i.e. screening means, which acts to maintain a desired gaseous atmosphere in the coiling apparatus . In the case of a conventional plant, the slab is coiled at a relatively high temperature in the coiling apparatus and stored there for some time for temperature homogenising or for waiting for further processing in the rolling apparatus. When a non¬ oxidizing atmosphere is provided the slab is prevented from oxidising or oxidising further during its stay in the coiling apparatus . The coiling apparatus preferably has sealing means, such as a door for closing its entry port and maintaining the desired atmosphere in it, when it is detached from the furnace apparatus.

Another feature which provides greater flexibility in use is that the coiling apparatus is provided with a mandrel onto which the coil can be coiled. The crop end of a slab, whether or not subjected to roughing, is clamped onto the mandrel and then coiled in the coiling apparatus into the coil in a path determined by the mandrel. This forced path makes it possible to coil a wide range of thicknesses reliably. This achieves a great freedom in the part of the process taking place prior to coiling, and it is also possible to coil thin, rolled slabs. Such slabs have a relatively 'large exposed surface. With the plant in accordance with the invention this surface is screened from oxygen from the outside atmosphere. Consequently it is possible to profit from the plant to the maximum.

The invention also provides a coiling apparatus and a furnace apparatus as described above, useful as components of the plant in accordance with the invention.

Introduction of the drawings The invention will be illustrated in the following by a non-limitative example of an embodiment, with reference to the drawings. In the drawings : -

Fig. 1 is a schematic top-view of a plant in accordance with the invention, and

Fig. 2 is a schematic side-view of the plant of Fig. 1.

Description of the embodiment Fig. 1 shows a continuous casting machine 1 for two strands . The continuous casting machine 1 comprises a ladle turret 2 in which two ladles 3 and 4 can be accommodated. Each of the two ladles can contain approximately 300 tons of liquid steel. The continuous casting machine is provided with a tundish 5 which is filled from the ladles 3 and 4 and kept filled. The liquid steel runs out of the tundish into two moulds (not drawn) from where the steel, now in the form of a partially solidified slab with its core still liquid, passes between the rolls of curved roller tables 6 and 7. For some grades of steel it can be an advantage to reduce the steel slab in thickness in roller tables 6 and 7 while its core is still liquid. This is known as squeezing. lϋ Descaling sprays 8 are located on the exit side of the two roller tables 6 and 7, by which oxide scale is sprayed from the slab with a water pressure of approximately 200 bar. Starting with a cast thickness of for example approximately 60 mm, the slab typically still has a thickness following squeezing of approximately 45 mm. By the 3-stand roll trains 9 and 10 the slab is further reduced to a thickness ranging from 10 to 15 mm. If desired the head and the tail may be cut off the slab by the shears 11 and 12, or the slab sheared into parts of a desired length.

Instead of casting a thin slab with a thickness of less than 100 mm, it is also possible to cast a thicker slab and by means of rolling, in particular by means of reversible rolling, to reduce the thickness of the slab to a value ranging from 10 to 15 mm.

For making hot-rolled steel the slab will generally be rolled into an intermediate slab with a thickness of 10 to 15 mm, as mentioned above. This rolled slab is conveyed into the furnace apparatus 13 or 14. The furnace apparatuses are each provided with heating means (not drawn) , for example induction heating means, for heating the rolled slab up ' to a desired temperature in the austenitic region. The furnace apparatuses are in the form of enclosures and are provided with conditioning means for creating and preserving a desired non-oxidizing gaseous atmosphere in the furnace apparatus . In the embodiment shown the conditioning means of a furnace apparatus comprise a suction line 15, a pump 17, gas metering and gas scrubbing means 19 and a supply line 21 along which the gas is pumped into the furnace apparatus. If desired the gas metering and gas scrubbing means 19 may also comprise a gas heating apparatus for compensating for any heat loss. Thus heat exchangers can be employed to control the gas temperature, using gas combustion to supply heat, and water for cooling.

The furnace apparatus is provided on its entry and exit sides with ports 23, 25 having sealing means to substantially prevent any undesired penetration of gas from the surrounding atmosphere. A suitable value for the temperature of the reduced slab on exiting the furnace apparatus is 1080"C. The furnace apparatus is coupled essentially gas-tightly to the coiling apparatus 27, which coiling apparatus 27 itself comprises an essentially gas-tight enclosure in which the slab is coiled into a coil. The coiling apparatus is preferably provided with a mandrel 29 which supports the coil as it is being coiled.

In this embodiment, the gas atmosphere provided in the furnace apparatus also enters the coiling apparatus when the latter is connected to it. Alternatively both the furnace apparatus and the coiling apparatus may be provided with conditioning means, as described above, for providing the desired atmosphere.

As appropriate, virtually synchronously with coiling of a slab onto coiling apparatus 27, a slab cast on the other strand is coiled in coiling apparatus 28 provided with a mandrel 30 (not drawn) . Coiling apparatuses 27 and 28 and furnace apparatuses 13 and 14 are each provided with sealing means 33, 35, 34, 36 respectively, by which the coiling apparatuses and the furnace apparatuses may be sealed for uncoupling, so that following uncoupling no gas can penetrate from the outside atmosphere and the gaseous atmosphere in the coiling apparatuses and the furnace apparatuses remains preserved. The sealing means for the ports of the furnace apparatuses and the coiling apparatuses are suitably steel flaps, biassed to the closed position, or they may be doors which are driven. To minimize gas leakage, flexible curtains may additionally be provided.

As soon as the coiling apparatus 27 is filled with a slab coiled into a coil, this coiling apparatus 27 is uncoupled from the furnace apparatus 13 and driven from position A (see Figure 1) past position B to position C. At position C there is a turnstile 31 (not drawn) by which at position C the coiling apparatus may be rotated through 180° around a vertical axis. Following rotation the coiling apparatus is driven past waiting position D to entry position E. As a coiling apparatus travels from position A to position E, an empty coiling apparatus is driven from position E to a turnstile 37 at position F. Following rotation through 180° around a vertical axis by the turnstile 37, the coiling apparatus is driven past position G to the starting position A and there it is ready for taking up a fresh slab.

A corresponding working method is applicable for the second strand, whereby the coiling apparatus 28 filled with a coil is driven from position B to position C and following 180° rotation to position D. The coiling apparatus stays parked in this position until a coiling apparatus which is currently uncoiling, for example coiling apparatus 27, is empty at position E and driven off to the now vacated position F. As soon as coiling apparatus 28 leaves position B, an empty coiling apparatus from position I, following rotation through 180° around a vertical axis by means of a turnstile 38, is moved via position K to take up the position of the coiling apparatus 28 now driven off. The new slab fed out of the furnace apparatus 14 can be coiled in the empty coiling apparatus. Devices, preferably electrical current conductors (not shown) , are fitted along the paths over which the coiling apparatuses travel for providing power for internally heating the coiling apparatuses according to need. For this purpose, the coiling apparatus contains electrical heaters for heating the coils and contacts for pick-up of power from the fixed conductors. Path B, C, D, E is common and used as described by coiling apparatuses of both strands. Position C has a rotation facility and position D is a waiting position in which a coiling apparatus filled with a coil is ready to be moved to position E as soon as it becomes free. Positions C and D may be swapped or may coincide.

In the manner described, a coiling apparatus 27 arrives at position E with its sealing means 33 closed and filled with a coil with a temperature of approximately 1080 °C. After the sealing means 33 have been opened the extremity of the outer winding corresponding to the tail of the coiled slab is fed into the rolling train. If desired the head may be cut off by crop shears if it does not have a suitable shape or composition for further processing. Should some oxide still have occurred, this can then be removed easily using the high pressure spray 42. In practice oxide formation will be negligible because the slab has been almost constantly in a conditioned gaseous atmosphere. Because the coiling apparatus rotates through 180°, its original infeed which is now the outfeed can be brought up very close to the entry of the rolling train. This also minimizes oxide formation. In the example shown, the rolling train 40 is provided with four mill stands and is so designed that the slab can be rolled in the austenitic range, or at least at such a temperature that only a small part converts to ferrite . A minimum target temperature of approximately 820 °C applies for low-carbon steel. For controlling thickness, width and temperature, a measuring and control apparatus 43 may be incorporated in the rolling train, after or between the mill stands.

As described above, the apparatus in accordance with the invention achieves the effect that less oxide forms as the slab and the strip are being processed. Because of this and because of the lower entry speed in the last rolling train 40 which this achieves as an additional advantage, it is possible to attain a smaller than conventional finished thickness of the hot rolled steel. Exit thicknesses of 1.0 mm and less from the rolling train 40 can be attained with the plant described. After exiting the rolling train 40, the hot-rolled strip passes through a cooling line 44 in which the strip is cooled to a desired temperature in the ferritic range by means of water cooling. Finally the strip is coiled into a coil on the coiling apparatus 45. By selecting the cooling on the cooling line it is possible in a manner known in itself to influence the recrystallisation in the ferritic range and thereby influence the mechanical properties of the hot rolled strip. Therefore, using the plant in this manner it is possible using the casting heat to manufacture in a successive series of process stages an austenitically rolled steel strip with good properties. External heating after casting may be avoided (except any heat generated by the rolling) .

After the hot-rolling and coiling on the coiling apparatus 45, the strip is further cold-rolled in a conventional manner if it is intended to be needed for making a formable steel which in turn is used to make can bodies by deep-drawing and ironing.

Claims

1. A method of manufacture of thin hot-rolled steel strip, comprising the steps of (i) forming a liquid steel into a cast slab having a thickness of less than 100 mm by means of a continuous casting machine, (ii) rolling said slab in the austenitic region into an intermediate slab having a lesser thickness than said cast slab, and

(iii) rolling said intermediate slab in the austenitic region into a strip, wherein said slab or said intermediate slab is for a period of time in a furnace in which a non-oxidizing atmosphere is maintained in contact with the surface thereof, and the thickness of said intermediate slab is in the range 5 to 25 mm and in step (iii) said intermediate slab is rolled to a final thickness of less than 1.2 mm.

2. A method according to claim 1 wherein said thickness of said intermediate slab is in the range 5 to 20 mm.

3. A method according to claim 1 or 2 wherein step (iii) is performed in a final rolling train, and the entry speed of said intermediate slab to said final rolling train is greater than 0.6 m/s.

4. A method according to claim 3 wherein said entry speed is more than 0.8 m/s.

5. A method according to any one of claims 1 to 4 including temporarily storing said intermediate slab as a coil in a coiling apparatus, prior to step (iii) .

6. A method according to claim 5 wherein in said coiling apparatus a non-oxidizing atmosphere is maintained in contact with said intermediate slab therein.

7. A method according to claim 5 or claim 6 wherein said intermediate slab is fed directly from said coiling apparatus into a hot-rolling train in which step (iii) is performed.

8. A method according to any one of claims 1 to 7 wherein between its continuous casting and the end of its rolling in step (iii) , the steel as a whole is not subjected to re-heating, apart from heat generated by the rolling.

9. A method of manufacture of a steel can body, wherein steel strip made by a method according to any one of the preceding claims is subjected to ferritic rolling, followed by deep-drawing and, optionally, ironing.

10. A plant for the manufacture of thin hot-rolled steel strip, comprising

(a) a continuous casting machine (1) for casting a steel slab,

(b) a furnace apparatus (13,14) arranged for receiving steel slab cast in said continuous casting machine, for adjusting the temperature of the steel slab, said furnace apparatus having an entry and an exit for the slab and an enclosed path for the slab from said entry to said exit,

(c) a coiling apparatus (27,28) for receiving the steel slab from said furnace apparatus, coiling the slab and subsequently uncoiling the slab, said coiling apparatus having an enclosure providing an enclosed space in which the slab is coiled,

(d) rolling apparatus (40) for receiving the steel slab uncoiled from said coiling apparatus and rolling the slab into strip of a desired thickness, and

(e) means (15,17,19,21) for providing a non- oxidising gas atmosphere in at least said furnace apparatus at said path thereof, wherein said exit of said furnace apparatus is substantially gas-tightly connected to said coiling apparatus.

11. A plant according to claim 10 wherein said coiling apparatus has at least one mandrel for coiling the slab thereon.

12. A plant according to claim 10 or 11, wherein said coiling apparatus has an entry port for said slab and sealing means for substantially sealing said entry port when not connected to said furnace apparatus.

13. A plant according to any one of claims 10 to 12 wherein said coiling apparatus is mobile, when disconnected from said furnace apparatus and containing a coiled slab.

14. A plant according to any one of claims 10 to 13 wherein said means for providing a non-oxidizing atmosphere comprise recirculation means for discharging said atmosphere from the furnace apparatus, optionally processing it and returning it into said furnace apparatus.

15. A plant according to any one of claims 10 to 14 wherein said furnace apparatus has means for cooling gas of said gas atmosphere therein.

16. A furnace apparatus (13,14) for use in a plant for the manufacture of steel strip, having an enclosure defining a path for traversing by a steel slab which is later to be rolled into strip, an entry (23) for entry of a steel slab into said enclosure, an exit (35,36) for exit of a steel slab therefrom, means (15,17,19,21) for providing a non-oxidising atmosphere in said enclosure at said path thereof, and means for gas-tightly connecting said exit to a coiling apparatus for said slab.

17. A coiling apparatus (27,28) for use in a plant for the manufacture of steel strip, having an enclosure defining a space, coiling means in said space for coiling of a slab which is later to be rolled into strip, an entry port into said space for entry of a slab thereto and means for gas-tightly connecting said entry port to an exit of a furnace apparatus for temperature adjustment of said slab.