GB2333987A - Method and apparatus for the manufacture of light gauge steel strip - Google Patents

Abstract

The invention relates to a method and suitable arrangement of apparatus used sequentially to produce light gauge steel strip. The feedstock is hot rolled steel strip (<10mm). It is intended as an upgrade to an existing process route comprising either a continuous casting and hot rolling or thin slab casting, homogenising and hot rolling. It's ideal usage is production of ultra light gauge strip of less than 1mm thickness. Strip exiting the existing process is cooled at a predetermined rate before coiling at a downcoiler 3 located part way down the existing runout table. The coil is rapidly transferred to an adjacent position where it is paid off and rolled in a high reduction stand 7, preferably using lubrication with the stock under tension. The temperature may be chosen to roll in either the austenitic, austenite + ferrite or ferritic region. The strip is then cooled at a prescribed cooling rate prior to being recoiled.

Description

Method and Apparatus for the Manufacture of Light Gauge Steel Strip The invention relates to a method and apparatus for the manufacture of light gauge steel strip.

Light gauge steel strip is conventionally produced via a process consisting of hot rolling and cold rolling, which may itself be followed by various downstream processes (e.g. pickling, annealing, coating).

Existing processes typically comprise a first step of producing a steel slab of about 200 mm in thickness for example by a continuous casting process and then hot rolling to a thickness of about 2 to 6 mm. This is usually achieved by a first rough rolling step reducing the slab to about 30 mm followed by a finish hot rolling step to form the finished hot rolled sheet of 2 to 6 mm thickness. Physical and economic limits currently restrict the production of lighter gauge strip directly off the hot mill (so demanding the need for a cold rolling step). The hot rolled strip is subsequently cold rolled to produce desired smaller thicknesses.

So-called 'ferritic' rolling has also increased in popularity over recent years during which time it has been practised on a conventional hot mill by either using low re-heating temperatures or by delaying the bar prior to entry to the finishing mill to allow the temperature to fall sufficiently for ferritic rolling. The former is necessarily restricted by operation flexibility during re-heating and by the gauge and width of material that can be rolled without overloading the roughing mill. The latter incurs a reduction in hot mill throughput during the delay period. Thus while the flexibility to offer a ferritically rolled product is desirable neither of the above solutions is ideal from a productivity point of view.

The production installations required for these hot rolling and cold rolling steps are very large and expensive to install.

In the methods disclosed in EP-A-0 226 446 a rolling process is described which attempts to provide a more compact installation which includes a warm rolling stage at the end of the hot rolling stage which rolls at speeds that must exceed 1500 m/min in order to develop the desired metallurgical properties. However a disadvantage of this method is that there is not much operational flexibility in the method in terms of developing the preferred metallurgical structure in the steel and there are also only a limited range of products with the desired range of metallurgical properties which can be produced. Furthermore, it is not practical to couple such high speed rolling to a casting process. It is a clear intention in this patent to de-couple the finish rolling and ferritic rolling steps (that are coupled EP-A-O 226 446) so that the rolling speeds are more sensible.

Rapid heat loss during conventional finishing has traditionally been a limiting factor in maintaining the steel in the austenitic condition and therefore obtaining the desired metallurgical properties. For instance, in the patent EP 0099520 A, the inclusion of an incubating device in the mill line is proposed to combat this problem. Clearly the aim of this invention is heat retention, and the ability to homogenise temperature to maximise yield and ensure austenitic finishing.

With the objective of streamlining rolling processes in terms of producing light gauge strip, the Patents WO92/22389A and WO92/00815A propose incorporating a rapid cooling device and 'cold' rolling facilities 'in-line' with a hot mill. The intention is very clearly to cool the strip sufficiently that from a metallurgical point of view, the material can be said to have been cold rolled.

It is therefore an objective of the invention to provide a method and apparatus for producing a wide range of light gauge steel strip with the desired metallurgical properties, by either ferritic or intercritical rolling.

It is a further objective of the invention to provide a method of producing light gauge steel strip in such a way that does not interrupt or reduce throughput of the hot strip mill, without the need for high speed rolling.

It is a further objective of the invention to produce a product in such a form that either eliminates later processing steps or leaves the material in a metallurgical condition such that it is of improved processability for later processing steps.

According to the invention there is provided a method and apparatus for the manufacture of light gauge steel strip comprising the following process steps sequentially: i) hot rolled steel strip, produced by any means, including at least one hot rolling stand, down to gauges of less than 1 0mm for production of soft material for later processing and preferably down to gauges of less than 7mm for production of light gauge hot band material, onto a run-out table, ii) after exit from the last hot rolling stand, the steel is cooled down to a predetermined temperature and/or at a predetermined rate on the run out table, before being coiled by a downcoiler located part way down the run-out table at a pre-determined point, iii) the coil is rapidly transferred by any suitable means to an adjacent position where it is paid off and threaded automatically or manually, iv) the material is rolled in either the single phase (austenitic), two phase (austenite/ferrite) or single phase (ferritic) states in at least one high reduction stand, vi) cooling of the strip to a prescribed temperature or at a prescribed cooling rate, and vii) coiling the strip.

There may be two high reduction stands are provided in tandem or even further stands or the stand may be a single reversing stand.

Lubricating means may be provided to provide lubrication of the stock-roll interface, and the strip may be rolled under tension by means of bridles before or after the mill, or both.

The invention for a method and apparatus for producing lighter gauge steel strip by hot rolling which either eliminates the cold rolling stage altogether or supplies the material in a more processable condition for cold rolling or other downstream processing will now be described in more detail with reference to the appended figures in which: Preferably variable cooling and heating means are optionally provided before and after the high reduction stand.

Fig. 1 is a schematic layout in elevation of a typical rolling mill incorporating the invention, Fig. 2 is a schematic layout in elevation of the method and apparatus of a first embodiment of the invention, Fig. 3 is a schematic layout in elevation of the method and apparatus of a further embodiment of the invention, Fig. 4 is a schematic layout in elevation of the method and apparatus of a further embodiment of the invention, Fig. 5 is a schematic layout in elevation of the method and apparatus of a further embodiment of the invention, Fig. 6 is a schematic layout in elevation of the method and apparatus of a further embodiment of the invention, Fig. 7 is a schematic layout in elevation of the method and apparatus of a further embodiment of the invention.

Fig. 8 is a schematic layout in elevation of the method and apparatus of a further embodiment of the invention.

In the manufacture of light gauge steel strip, the method and apparatus with which the invention is concerned are shown in figure 1 and performs the following steps sequentially: i) hot rolled steel strip produced, preferably in a multi-stand tandem finishing mill, but may also be produced by Steckeling or other means (1) down to gauges of less than lOmm for production of soft material for later processing and preferably down to gauges of less than 7mm for production of light gauge hot band material. ii) after exit from the last hot rolling stand, the steel is cooled in a controlled manner preferably using either air or water (2) down to a predetermined temperature before being coiled by a downcoiler or up coiler (3) which is preferably located part way down the run-out table at a suitable distance to enable use of the existing run-out table cooling system. iii) the coil is rapidly transferred by any suitable means to an adjacent position where it is paid off (4) and threaded automatically or manually through all or any combination of the arrangement of equipment stages in the various embodiments described below, to the downcoiler (12).

In the subsequent stages the material is rolled in either the single phase (austenitic), two-phase (austenite/ferrite) or single phase (ferritic) states. To allow flexibility of entry temperature, rapid heating (6) or cooling equipment 5 preferably using induction heating or water cooling respectively, may optionally be located either prior to or after an entry bridle (10), which is used to apply tension to the strip. Prior to entry into the subsequent rolling stage, the strip edges are preferably reheated by induction heating (14) to compensate for temperature loss during transfer, threading and rolling.

The material is then rolled in a high reduction stand or stands (7), preferably using lubrication of the stock-roll interface (13) and preferably with the stock under tension using entry and exit bridles (10).

The flatness or shape of the strip may optionally be measured by a flatness sensor (9) which is fed back to a backup roll (8) which can be used to control shape on-line.

Controlled cooling facilities on the outgoing side of the mill, preferably using air or water (11) enable the strip to be cooled to a prescribed temperature or to a prescribed cooling schedule. An option to use strip immersion, preferably in water, for rapid cooling on certain grades (15).

The strip is finally coiled at a coiling stage (12).

The invention is suitable for the production of a wide range of steel strip products but galvanised steel flanging and exposed automobile parts are J examples which demonstrate the breath and versatility of the invention.

Wide strip may also be called sheet, but for convenience the term 'strip' only, will be used. The term strip is used to describe the physical dimensions of the product and does not restrict the inventions use to grades of steel which are typically called 'strip products'. The method and apparatus of the invention may also, for example be adjusted to enable production of so-called 'pipe' and 'structural' grades which include HSLA steels or other compositions including silicon steels and various types of stainless steel.

Hot rolled steel strip is conventionally produced by multi-stand tandem finishing mill, but may also be produced by Steckeling or other means.

Further reduction of the strip down to thicknesses of between 0.5 and 1 .5mum is usually carried out by cold rolling. All such known and proposed hot rolling processes typically operate at the minimum gauges given above due to a number of physical restrictions (such as the mechanical limits, limits speed of strip and metallurgical constraints) but also governed by economic viability in terms of process throughput.

For this invention the prior production of hot strip down to gauges of less than lOmm and preferably less than 7mm is assumed and by any processing method, of which two examples of which were given above. The in-going strip thickness is designed to be typically 6mm for production of soft hot band. For production of light gauge hot band an in-going strip thickness of preferably less than 2mm would be typical since the greatest benefits of the invention will be found when the exiting hot strip material is of significantly less than lrnm.

The cycle time of the high reduction mill is preferably not designed so that all material is re-directed from the hot strip mill, but a variable proportion of the total HSM throughput. In the present invention the speed of rolling may be adjusted to match the desired fraction of product which is diverted from the existing hot mill process.

In contrast to EP-A-226466, which suggest a limit on the range of rolling speeds, the current invention the rolling speed is not restricted. The invention enables lower speeds to be used by virtue of the fact that the temperature and lubrication conditions may be varied over a wide range enabling production of a product with suitable properties for deep drawing application. In fact, the precise mechanical and physical properties of the end product may be deliberately controlled by altering the proportion of the phases present (predominantly austenite and ferrite although may be other phases) during the high reduction rolling process. This is preferably (and most conveniently) achieved by controlling the rolling temperature through the high reduction stand although adjustment of the alloy composition or other means may be used.

On exit from the existing processing equipment the strip travels down the run-out table to an additional downcoiler which is located part way down the existing run-out table. This downcoiler is located after a desired length of water cooling section so that the strip may either be air cooled or water cooled to a desired temperature prior to coiling using this new downcoiler and hence transferred to the high reduction stand(s) at any desired temperature.

During cooling on the run out table prior to transfer to the high reduction stand(s), a temperature profile may be deliberately developed along the length of the strip to offset the effects of non-uniform temperature loss by the coil during transfer and payoff into the high reduction stand(s). The coil is then rapidly transferred to the adjacent position by any suitable means, although a convenient means is to use a so called 'coil car'. In order to facilitate the speed of transfer and ease of paying off, it is preferable to use the coiler mandrel to act as the payoff reel, although transfer to a separate mandrel may also be possible. The coil would then preferably be threaded automatically through all or some of the rolling equipment to the downcoiler.

During transfer, threading and rolling the edges of the strip will cool in preferentially which may lead to excessive temperature loss prior to rolling and consequential loss in ductility. To over come this problem, preferred variations of the invention include the facility for heating the edges of the strip. The preferred method of edge heating is by induction heating although other methods may be used.

The method of the invention offers the opportunity to control the precise temperature of high reduction rolling through both the use of existing run out table cooling and the possibility of either further controlled cooling or reheating of the strip preferably by induction heating prior to entry to the high reduction stand. The ability to re-austenitise the material prior to high reduction rolling for instance, enables a wider range of products to be rolled and/or wider range of material properties to be achieved. The ability to further cool the strip may for example, be used to roll so-called 'interstitial free' steels to generate improved texture, by lubricated rolling at lower temperatures.

In one variation of this invention the temperature of the incoming strip is monitored and used to control either the amount of induction heating unit or controlled cooling to enable a uniform temperature or desired temperature profile to be achieved along the length of the stock when it arrives at the roll bite.

The arrangement of rolls within the stand or stands of the invention are designed to give maximum reduction in gauge in as few passes as possible and in the preferred variation of the invention a single stand will be used to take a reduction of at least 25% but typically 40% or above in a single pass.

A simple 4-high configuration or so-called 'Z-high' configuration of rolls are preferred, although any suitable means may be used and a planetary mill may also offer advantages.

To maximise the size of reduction that is possible, entry and exit bridles (10), enable both front and back tension to be applied to the stock during rolling. The bridles are located as close to (either side of) the mill stand as possible to minimise the amount of yield loss from rolling without tension.

Another feature which enables higher reductions to be taken is the use of lubrication, although this feature is also important for metallurgical reasons.

The application of lubricating oil to the work rolls or indirectly via application to the backup rolls, may be performed by any suitable method which will give uniform coverage the roll, but might typically be by jetting an oil/water mixture.

To avoid difficulties with threading the high reduction mill, the lubrication system (13) is not initiated until the first lap of the coil has been threaded into the exit bridle (10). The lubrication equipment (13) is then turned on as the mill (7) is accelerated up to full operating speed. This minimises the amount of strip that is rolled without tension/lubrication and ultimately minimises yield loss from the process.

To maximise the life of the work rolls and maintain optimum surface quality for as long as possible during rolling, convention roll cooling is applied by jetted water impingement using a conventional wiper system to avoid disruption of the lubricant film by cooling water.

To maximise the range of strip widths over which good shape can be maintained, one variation of the invention incorporates the use of a backup roll which may be used to control shape. The preferred method of backup roll shape control is to equip the stand(s) with a dynamic shape roll or DSRbackup roll (8), with hydraulically adjustable profile along the length of the roll. This enables the invention the ability to produce strip with good shape even up to very wide widths of 1500mum or more. A further variation of the invention uses a shape measuring roll (9) to feed back a control signal to the DSR backup roll. The preferred method is to use a PANICIMs type shape measuring roll. The shape measuring roll is moved into position once the exit bridle (10) has been threaded to avoid damaging it.

In contrast to the method described in EP-A-226466 and in order to develop a preferred metallurgical structure and to maximise the range of products and mechanical properties that may be produced, an embodiment of the method of the present invention includes a rapid cooling device is used for certain products. This is located between the mill stand(s) and the down coiler.

The invention enables a wide range of cooling rates to be produced after rolling. Use of a rapid water cooling system permits simple air cooling when 'off' with any cooling rate in between up to a maximum when operated at maximum output. The preferred method of water cooling is by the use ofwaterjets such as Davy jets but other types may be used.

In one variation of the invention an option to use immersion of the strip preferably in water enabling rapid cooling is preferable on certain grades.

The precise method is optional but might involve using an adjustable roll to immerse the strip to different depths in a water bath which would enable variation of the time the strip spends immersed and hence the degree of cooling. The water temperature may be adjusted to enable a wide range of cooling rates to be obtained and water near its boiling point may be used to remove large amounts of heat very rapidly.

It is beneficial to ensure the proximity of the downcoiler as close to the mill stand(s) as possible which thus constrains the distance in which controlled cooling of the stock can be achieved. An alternative rapid cooling method using the ADCOX cooling process, involves using a fan to drive a high pressure water/air mixture at the stock so enabling a wide range of heat transfer coefficients and wide range of cooling rates to be achieved. An further alternative variant of the invention uses forced air cooling as an alternative cooling method for some products which may be fitted in place of or in addition to other cooling equipment.

One technique of improving metallurgical structure which leads to good formability steel strip is control of the degree of carbon precipitation, which determines the aging behaviour. The invention takes advantage of this by offering a wide range of processing conditions and in particular enabling control of the rate of cooling following rolling.

A further contribution to the development of a desirable metallurgical structure is the ability to control the process of recrystallisation which in turn controls the drawability of the product. Drawability is measured by the so-called r-value or Lankford value which is a measure of the crystallographic texture of the product. A number of aspects of the invention address the need to be able to control and develop a desirable rvalue in the product for good drawability, but three of these are discussed in particular.

The first is that high rolling reductions (large deformations) are taken which generate a high dislocation density in the product and hence a large driving force for recrystallisation. In conjunction with control of the rolling temperature this permits a wide range of processing conditions with lower temperature, high reduction rolling being favourable for development of high r-value. It is possible to use the invention to produce unrecrystallised strip and use a downstream process such as hot dip galvanising to control recrystallisation.

The second aspect is that lubrication may be used during the high reduction rolling - process. Without the use of lubrication an undesirable crystallographic texture may be developed in the surfaces of the material in contact with the roll. Lubrication of the rolling process is highly desirable for production of high r-value product. The third aspect of developing high r-value material is the ability to control the rate of cooling of the product after high reduction rolling between simple air cooling and rapidly water cooling. A slow air cool may be used to enable the product to 'self anneal' on cooling from high temperature so producing a softer end product. Rapid cooling offers a wide range of possibilities such as the ability to 'freeze in' the as rolled structure, or develop controlled amounts of 'second' phase, in the product, such as bainite.

Due to high reduction taken on the stand(s) and the wide range of processing speeds available, the exit speed of the strip may vary considerably but in particular reach very high speeds when producing light gauges. Rather than have to thread the downcoiler before accelerating the mill, which incurs a large yield loss, a shear and downcoiler combination enables threading of strip at up to 20m/s. With this combination of shear and downcoiler and associated strip guiding equipment full speed guiding of a newly sheared strip end on to the mandrel can be achieved owing to the proximity of the mandrel to the shearing plane and to the special guiding equipment incorporated therein.

Referring to fig. 4 an embodiment of the method and apparatus of the invention is shown for light gauge ferritic rolled formable hot band. The austenitic finishing stage is carried out at between 870-900 C (difficult at thinnest gauges) and the transformation to ferrite takes place during transfer which takes about one minute. The strip is then deformed at approximately 750"C, and coiled at between 675-700"C to self anneal. The product is formable but not drawable. Uses for this type of steel are as a substitute for cold rolled product, after skin passing for surface finnish, or as a substrate for hot dip galvanising. Inductive heating of the edges of the strip and/or to control the temperature uniformity along the length of the strip may be used to maximise product yield.

Referring now to fig. 5, an embodiment is shown for dual-phase steels. In this embodiment, the steel is transferred in partial transformed (ferritic) condition. Consistency of the cycle times are essential to control the fraction transformed from coil to coil. The strip is rolled at relatively high temperature (e.g. 700-750"C). High intensity cooling is required after rolling to quench in the 'second phase' be it martensite or ferrite.

Referring to fig. 6 a further embodiment of the method and apparatus of the invention is shown for the production of drawable IF steel hot rolled product. The austenitic finishing stage is carried out at 870-9000C (must be above 870"C to achieve good drawability), and the transformation to ferrite occurs during transfer of the coil in approximately 1 minute).

In order to develop good drawability, the lower the temperature of rolling the better, so it is necessary to have additional cooling prior to the high reduction stand (7). The deformation may be anywhere between 400 and 750"C by water cooling (5) of strip both prior to coiling and between the payoff reel (4) and the mill (7, 8). Various cooling rates may be used depending on the subsequent processing of the strip. The resulting strip product is highly drawable and may be used for demanding, drawable applications.

Referring now to fig. 7 an apparatus arrangement and method is shown for the production of steels where modification of the metallurgical structure and refinement of the grain size is desired, which may be achieved by reaustenitisation. Rapid reheating is used prior to the high reduction mill. The strip is then rolled and the subsequent cooling controlled to generate the desired properties. The steel produced by this method has high strength and toughness.

Figure 8 shows an apparatus arrangement enabling immersion of the steel strip for rapid heat removal permitting the production of steels with a range of metallurgical properties, especially with dual-phase structure.

Claims (1)

1. Claims (1) Method of manufacture of light gauge steel strip comprising the following process steps sequentially:

   i) hot rolled steel strip, produced by any means, including at least one hot rolling stand, down to gauges of less than 1 0mm for production of soft material for later processing and preferably down to gauges of less than 7mm for production of light gauge hot band material, onto a run-out table,

   ii) after exit from the last hot rolling stand, the steel is cooled down to a predetermined temperature and/or at a predetermined rate on the run out table, before being coiled by a downcoiler located part way down the run-out table at a pre-determined point,

   iii) the coil is rapidly transferred by any suitable means to an adjacent position where it is paid off and threaded automatically or manually,

   iv) the material is rolled in either the single phase (austenitic), two phase (austenite/ferrite) or single phase (ferritic) states in at least one high reduction stand,

   vi) cooling of the strip to a prescribed temperature or at a prescribed cooling rate, and

   vii) coiling the strip.

   (2) A method according to claim 1, characterised in that at least two high reduction stands are provided in tandem.

   (3) A method according to claim 1, characterised in that the high reduction stand is a single reversing stand.

   (4) A method according to claim 1, characterised in that a lubricating means is provided to provide lubrication of the stock-roll interface.

   (5) A method according to claims 1, characterised in that the strip is rolled under tension.

   (6) A method according to claim 1, characterised in that the strip is rolled at the high reduction stand in the ferritic phase at approximately 750 "C.

   (7) A method according to claim 1, characterised in that the location of the downcoiler with respect to the run-out table is adjustable.

   (8) A method according to claim 1, characterised in that additional cooling means are provided before the high reduction stand which provide variable cooling rates to develop any desired temperature profile along the length of the strip prior to rolling.

   (9) A method according to claim 1, characterised in that induction or other heating equipment is provided before the high reduction stand to heat the strip at any desired rate or to any desired temperature profile, to allow flexibility of entry temperature to the high reduction stand.

   (10) A method according to claim 9, characterised in that strip edge heating means are provided before the high reduction stand.

   (11) A method according to claim 9, characterised in that the induction or other heating equipment is provided after the entry bridle.

   (12) A method according to claim 9, characterised in that the induction or other heating equipment is provided before the entry bridle.

   (13) A method according to claim 1, characterised in that variable lubrication means are provided at the high reduction stand to facilitate the degree of reduction that can be achieved and metallurgical quality.

   (14) A method according to claim 1, characterised in that the high reduction stand comprises a shape controlling backup roll to control shape of the strip.

   (15) A method according to claim 14, characterised in that a shape measuring device is provided to measure the shape of the strip and provide input data to the shape controlling back up roll.

   (16) A method according to claim 1, characterised in that the high reduction mill is arranged approximately in-line with the existing hot strip mill and the corresponding throughput speeds are matched with a fast downcoiler arranged at the end of the run out table to coil the strip from the last hot rolling stand.

   (17) A method according to claim 1, characterised in that additional cooling means are provided after the high reduction stand which provide variable cooling rates to develop any desired temperature profile along the length of the strip prior to rolling.

   (18) A method according to claims 8 and 17, characterised in that the water cooling means are provided by immersion of the strip in a bath of coolant.

   (19) An apparatus in accordance with the method in any one of the preceding claims.