EP3941655B1 - System and method for producing metal hot-rolled strip - Google Patents

Description

The present invention relates to a system and a method for producing metallic hot strip with improved mechanical properties due to a fine grain structure of the structure.

During hot rolling in a rolling mill, warm rolling stock is plastically deformed in a roll gap between two work rolls of a rolling stand, thereby reducing the thickness of the rolling stock. After hot rolling, the rolled stock is typically cooled in a cooling train and then conveyed out of the rolling train, for example as coils or sheets.

The material properties of a hot-rolled strip not only depend on its chemical composition, but also depend to a large extent on the timing of the processing steps in the rolling mill. When setting the structure and phase components of the rolled strip, what is particularly important is the time interval between the last rolling pass in a rolling stand of the rolling train and the start of cooling of the strip. In many cases this period should be as short as possible.

Hot rolling mills usually have a rolling train with a fixed number of rolling stands. It is generally not possible to remove one or more rolling stands when producing thick strips and add them again when producing thin strips.

Thick strips experience a smaller reduction in thickness in a rolling mill than thin strips. As a result, a thick strip is often finished rolling before the last rolling stand, whereas a thin strip in the same rolling mill is only finished in the last one Roll stand is finished rolling. During its production, the thick strip passes through the nth rolling stands of a rolling train unrolled.

Due to the relatively slow transport speed of thick strips and the fact that the last forming of these strips takes place before the last rolling stand, there is a relatively long period of time until the finished rolled strip reaches the cooling section behind the last rolling stand and is cooled there with a certain intensity . This long period of time can mean that thick strips can no longer achieve certain material properties.

Based on this, the EP 3 434 383 A1 A stand cooler was proposed, which can be installed in place of the work rolls of a roll stand that are not required for thick strips.

It has been shown that when the known stand cooler is used in the last rolling stand, the hot strip is often only cooled down after a few seconds. This means that a sufficiently fine grain structure of the structure cannot regularly be achieved. In addition, the system availability is disadvantageously reduced due to the conversion effort.

Another system and another process for hot rolling steel strip is in the DE 10 2013 107 010 A1 described. The known system has a hot rolling train, which comprises several rolling stands to be passed through one after the other in the conveying direction of the steel strip to be hot-rolled, and a cooling section for intensive cooling of the hot-rolled steel strip emerging from the last rolling stand of the rolling train. The beginning of the cooling section, viewed in the conveying direction of the steel strip to be hot-rolled, is moved before the end of the hot-rolling train and the cooling section begins after the last rolling stand passed through, in which the steel strip to be hot-rolled is hot-rolled.

In the known system, compact cooling units are used, each of which applies a jet of cooling fluid concentrated to a specific section onto the hot strip. The length measured in the conveying direction of the steel strip to be hot-rolled, via which the cooling unit, which is arranged behind one of the rolling stands within the rolling train in the conveying direction, applies cooling fluid to the steel strip, in the known system at a maximum of 25% of the distance at which the rolling stands of the rolling train, which are arranged adjacent to one another, are set up one after the other in the conveying direction , limited. Due to the limitation of 25% of the distance, very large quantities of cooling water have to be transported in the immediate vicinity of the scaffolding. The exposure time of the cooling fluid is very limited. Due to the limited exposure time, there is a drop in temperature on the belt surfaces, but the interior of the belt does not cool down sufficiently. The immediate reheating prevents a sustained drop in temperature. For this purpose, spraying devices are provided in the known system, which direct a high-pressure jet directed transversely to the conveying direction in the direction of the respective cooling unit at least onto the top side of the steel strip in order to drive cooling fluid standing there from the surface in question.

The according to DE 10 2013 107 010 A1 The required side spray devices make the construction and control of the known cooling device relatively complex. In addition, a relatively high proportion of the coolant exits laterally, so that appropriate additional splash protection devices must be provided.

Based on what is in the publications DE 10 2013 107 010 and EP 3 434 393 A1 Known prior art, the present invention is therefore based on the object of providing a versatile system for producing metallic hot strip with improved mechanical properties due to a fine grain structure of the microstructure and a corresponding method.

The WO 2011 / 138 159 A1 discloses a device and a method for hot rolling steel strips in several successive rolling stands, wherein the steel strips are first rolled in the austenitic state in one or more rolling stands and then, after liquid cooling, in the ferritic state to the final thickness. In order to ensure that the steel strip actually reaches the ferritic state after cooling, it is intended that the final thickness of the steel strip is less than 3 mm. The difference between the The exit temperature of the steel strip from the last rolling stand before the liquid cooling and the equilibrium austenite limit temperature is set by the pilot control or regulation of the exit temperature to a maximum of 70 K, preferably not more than 50 K, preferably less than 25 K.

The publication DE 10 2013 019 698 A1 discloses a method for producing a metallic strip, in which the strip is rolled in a multi-stand rolling mill, is discharged behind the last rolling stand of the rolling mill in the conveying direction and is cooled in a cooling device. In order to achieve a favorable grain structure and a high degree of flatness, the invention provides that the strip or sheet is subjected to additional rapid cooling immediately after passing the work rolls of the last rolling stand, with the cooling of the strip or sheet at least partially still occurring within the extension of the last one Roll stand takes place in the conveying direction, the rapid cooling taking place by applying a cooling medium to the strip or sheet from above and below, the volume flow of cooling medium applied from below to the strip or sheet being at least 120% of that applied to the strip or sheet at the top Volume flow of cooling medium is. The object on which the invention is based is solved by the subject matter of the main and secondary claims. Advantageous refinements are specified in the subclaims.

What is proposed is a system for producing metallic hot strip with a hot rolling train for carrying out one or more rolling passes, having at least one nth rolling stand in the conveying direction of the hot strip and at least one nth rolling stand upstream in the conveying direction of the hot strip, with a cooling device, wherein the Cooling device has at least one first upper rapid cooling spray bar for applying a cooling medium to the top of the hot strip between the n-1'th roll stand and the n'th roll stand, wherein the cooling device has at least one first lower rapid cooling spray bar for applying the cooling medium to the underside of the hot strip between the n-1'th roll stand and the n'th roll stand, and wherein the cooling device covers at least 30 percent of the length L of the hot strip is set up with the cooling medium between the n-1'th roll stand and the n'th roll stand.

Covering >30% of the length L of the hot strip between the n-1'th and n'th rolling stand has the advantage that the cooling can take place both quickly and continuously, so that there is no renewed grain growth between different cooling sections can.

The first upper and/or the first lower rapid cooling spray bars are arranged immediately behind the outlet of the n-1'th or n'th roll stand in order to cool the hot strip immediately after it leaves the roll gap on the outlet side.

Rapid cooling spray bars are understood to mean cooling devices that are set up to deliver a large amount of a cooling medium under high pressure for rapid cooling of the hot strip. They differ, among other things, from the cooling devices used to cool the rolls of the rolling stands.

The cooling medium can in particular be water, which is usually already available in large quantities in a plant for producing metallic hot strip. The high heat capacity of water can allow the hot strip to cool down particularly quickly.

The nozzle outlet from the rapid cooling spray bars is designed in such a way that the emerging coolant can act on the hot strip immediately after leaving the roll gap on the outlet side, so that the coolant flow from a distance of less than 500 mm, preferably less than 400 mm, particularly preferably less than 350 mm after leaving the roll gap on the hot strip and the cooling of the hot strip occurs immediately after leaving the n-1'th roll stand. The rolling train has a tracking system, whereby the tracking system tracks the position of the hot strip and the rapid cooling control device receives a signal to start as soon as the start of the hot strip passes through the roll gap and a time-delayed signal to stop is given as soon as the hot strip has passed has left the roll gap, so that the cooling of the hot strip occurs immediately after leaving the n-1'th roll stand.

The term "behind" refers to the direction of material flow.

In a first embodiment of the system for producing metallic hot strip, the upper rapid cooling spray bar and/or the lower rapid cooling spray bar (SK1u) is set up to spray the cooling medium at a pressure of 1 to 20 bar, in particular 3.0 to 10 bar. particularly 3.0 to 5 bar. The application of the cooling medium at a pressure of 1 to 20 bar facilitates particularly rapid cooling of the strip through a turbulent fluid flow on the top and bottom of the strip. Dispensing the cooling medium at a pressure of 3.0 to 10 bar, in particular 3.0 to 5 bar, can simplify the control of the cooling device.

, wobei d die Banddicke des Warmbandes ist. So ergibt sich beispielsweise bei einer Banddicke von 10 mm und einer spezifischen Abkühlrate von 600 K/s pro mm eine Kühlrate C_R von 60 K/s.A further embodiment of the system for producing metallic hot strip provides that the cooling device is set up to cool the hot strip with a specific cooling rate C _Ri of at least 300 K/s per mm of strip thickness, preferably 600 K/s per mm of strip thickness, in particular with a specific cooling rate of more than 900 K/s per mm of strip thickness. A specific cooling rate of more than 600 K/s per mm of strip thickness leads to a particularly fine grain structure of the structure of a hot steel strip produced with the proposed system. Accordingly, the steel strip can have improved strength and toughness properties. The mechanical properties of the metallic hot strip or sheet produced can be further improved if the cooling device is set up for a specific cooling rate of more than 900 K/s per mm of strip thickness. The absolute cooling rate C _R is determined from the formula $C_R=\frac{C_{\mathit{Ri}}}{d}$

, where d is the strip thickness of the hot strip. For example, with a strip thickness of 10 mm and a specific cooling rate of 600 K/s per mm, this results in a cooling rate C _R of 60 K/s.

Furthermore, an exemplary embodiment of the system for producing metallic hot strip is proposed, in which the cooling device has more than one upper rapid cooling spray bar and/or more than one lower rapid cooling spray bar for applying the cooling medium to the top or bottom of the hot strip. The upper rapid cooling spray bars are intended for applying the cooling medium to the top side and the lower rapid cooling spray bars are intended for applying the cooling medium to the underside of the hot strip. The use of multiple rapid cooling spray bars can further simplify the control of the cooling device. In particular, the cooling rate can be varied in the conveying direction of the hot strip to achieve an optimal structure.

In another embodiment of the system for producing metallic hot strip, the cooling device has at least one upper rapid cooling spray bar arranged after the nth rolling stand for applying the cooling medium to an upper side of the hot strip, which is identical in construction to the first upper rapid cooling spray bar. Alternatively or additionally, the cooling device has at least one lower rapid cooling spray bar arranged after the nth roll stand for applying the cooling medium to the underside of the hot strip, which is identical in construction to the first lower rapid cooling spray bar. The use of identical rapid cooling spray bars after the n-1'th and n'th rolling stands simplifies the design of the hot rolling train. In addition, the effort associated with maintaining the system can be reduced.

A further embodiment of the system for producing metallic hot strip provides that the system has a process computer for determining the amount of cooling medium and / or distributing the amount of cooling medium to the rapid cooling spray bar or beams. With the process computer, depending on the chemical composition and/or the dimensions of the metallic material to be rolled, the cooling rates and cooling courses can be specifically adjusted in order to favorably influence the structure of the hot strip and its mechanical properties. The process computer can with one after the nth, in particular The measuring device arranged on the last rolling stand can be connected in order to regulate the amount of cooling medium and/or distribution of the amount of cooling medium to the rapid cooling spray bar or beams during operation depending on the values measured by the measuring device, for example the temperature of the hot strip.

Furthermore, an exemplary embodiment of the system for producing metallic hot strip is proposed, in which the n-1'th rolling stand is set up to carry out the last rolling pass. The cooling device can therefore be used to enable very rapid, early cooling of the hot strip directly after the last rolling pass, so that a very fine structure with corresponding advantageous material properties can be obtained. Furthermore, the n'th roll stand, with which no further thickness reduction is carried out, can shield a measuring device arranged behind the n'th roll stand from/from the cooling medium discharged with the cooling device. The risk of an influence on the measurement and/or damage to the measuring device due to the large quantities of cooling medium required for rapid cooling can thus be reduced.

In another embodiment of the system for producing metallic hot strip, the rolls of the nth, in particular the last, roll stand are used as squeezing rolls or as drive rolls. The rolls of the nth roll stand can therefore significantly reduce the amount of cooling medium on the hot strip. In addition, by using the rolls of the nth roll stand as squeezing rolls or drive rolls, the hot strip can be continued after the nth rolling pass and the strip run can be stabilized. This can further increase the quality of the hot strip produced.

In another embodiment of the system for producing metallic hot strip, the rolls of the nth roll stand are used to carry out a skin pass with an extension of less than 10% (preferably less than 3%) in order to eliminate possible flatness errors.

A further embodiment of the system for producing metallic hot strip provides that at least one of the upper and lower rapid cooling spray bars is arranged extendable. This can simplify maintenance in the area between the rolling stands. The system can, for example, be designed in such a way that the rapid cooling spray bars can be extended laterally, ie in the plane of the hot strip and perpendicular to the conveying direction. In principle, it is also conceivable to design the system in such a way that the rapid cooling spray bars can be extended in a direction perpendicular to the hot strip.

In a further embodiment of the system, the cooling of the rolling stock or the hot strip begins immediately spatially and temporally after leaving the n-1'th rolling stand.

For this purpose, the nozzle outlet from the rapid cooling spray bars is designed in such a way that the emerging coolant can act on the hot strip immediately after leaving the roll gap on the outlet side. The coolant flow acts on the rolling stock from a distance of less than 500 mm, preferably less than 400 mm, particularly preferably less than 350 mm after leaving the roll gap.

In terms of time, rapid cooling also begins immediately when the n-1st roll stand is left. The position of the hot strip is tracked via a tracking system. As soon as the beginning of the strip passes through the roll gap, the rapid cooling control device receives a signal to start. A time-delayed stop signal occurs as soon as the rolling stock or hot strip has left the rolling gap.

Furthermore, a method for producing metallic hot strip using a system described above is proposed. The use of the proposed system can enable the production of metallic hot strip with a particularly fine structure and improved mechanical properties.

The invention is explained in more detail below using exemplary embodiments. It shows schematically:
Fig. 1 a plant A for the production of metallic hot strip B.

The system A for the production of metallic hot strip B includes a hot rolling train for carrying out one or more rolling passes. The hot strip B is conveyed in the conveying direction F through the n roll stands, in particular the n-1'th roll stand and the n'th roll stand. In the present exemplary embodiment, the stand n-1 denotes the penultimate roll stand and n the last roll stand.

In further exemplary embodiments, not shown, n can be the 4th to 8th rolling stand of a rolling train.

The n-1'th rolling stand is set up to carry out the last rolling pass.

There are preferably further assemblies between the n-1'th and n'th roll stands, in the so-called intermediate stand area. These are known to those skilled in the art. In the figure, roller cooling systems, belt guide tables, loopers, and side guides are shown schematically in a dash-dotted line to show the limited space. A simple geometric adjustment of assemblies behind a final rolling stand in the intermediate stand area is not possible

The system A also has a cooling device which is in the Fig. 1 Embodiment shown includes four upper rapid cooling spray bars SK1o, SK2o, SK3o, SK4o and four lower rapid cooling spray bars SK1u, SK2u, SK3u, SK4u. However, it is also conceivable to provide more or fewer rapid cooling spray bars. In particular, the number of rapid cooling spray bars which apply cooling medium W to the top side O of the hot strip B can differ from the number of rapid cooling spray bars which apply cooling medium W to the underside U of the hot strip B. It is also conceivable that the design of an upper rapid cooling spray bar differs from the design of a lower rapid cooling spray bar, as stated in the Fig. 1 is indicated in relation to the pair of rapid cooling spray bars SK3o/SK3u.

The rapid cooling spray bars SK1o, SK2o, SK3o, SK4o, SK1u, SK2u, SK3u, SK4u shown are designed to deliver cooling medium with a pressure of 1 to 20 bar, in particular 3.0 to 10 bar, very particularly preferably 3.0 to 5 bar. The cooling medium W used in the system A shown is water, which is typically available in large quantities in a rolling mill. In order to enable rapid cooling of the hot strip, the cooling device is set up to cover at least 30 percent of the length L of the hot strip B between the n-1'th roll stand and the n'th roll stand with the cooling medium W.

According to the illustration in the figure, it is clear that the spray area of the rapid cooling SK10 and SK1u is as close as possible to the roll gap on the outlet side.

By means of the cooling device, the hot strip B can be cooled with a specific cooling rate of more than 600 K/s per mm of strip thickness, in particular with more than 900 K/s per mm of strip thickness. To cool the hot strip B, all four pairs of rapid cooling spray bars SK1o/SK1u, SK2o/SK2u, SK3o/SK3u, SK4o/SK4u are used here. In many cases, however, it can also prove to be expedient to carry out the rapid cooling of the hot strip only with rapid cooling spray bars arranged between the nth roll stand and the nth roll stand. In this way, the amount of cooling medium applied after the nth roll stand can be reduced.

The pair of rapid cooling spray bars SK1o/SK1u provided for cooling the hot strip B directly behind the roll gap of the n-1'th roll stand is identical in construction to the pair of rapid cooling spray bars SK4o/SK4u in the exemplary embodiment shown.

The system A has a control device S and a process computer P for determining the amount of water and distributing the amount of water to the rapid cooling spray bars SK1o, SK2o, SK3o, SK4a, SK1u, SK2u, SK3u, SK4u. In the exemplary embodiment shown, the process computer P accesses measurement data from a measuring device M, which is arranged shortly behind the nth, in particular behind the last, rolling stand of the rolling train. By means of the measuring device, for example, the temperature and temperature distribution, the thickness of the hot strip and the thickness profile of the hot strip transverse to the transport direction can be determined. The Measuring device M can, for example, be designed as a so-called measuring house. The nth roll stand shields a large part of the cooling medium M for cooling the hot strip from the measuring device M. For this purpose, the rolls of the nth roll stand can be used as squeezing rolls or drive rolls and remove the cooling medium located on the hot strip B. By using the rolls of the nth roll stand as squeezing rolls or drive rolls, the strip run can also be stabilized.

The control device S in the form of a tracking system is connected to the rolling stand in terms of signals in order to start the action of the coolant on the hot strip upon receipt of the signal that the hot strip is in the roll gap and to start the action of the coolant on the hot strip upon receipt of the signal that the hot bath has left the roll gap to stop the effect of the coolant on the hot strip with a time delay.

The rapid cooling spray bars SK1o/SK1u, SK2o/SK2u, SK3o/SK3u, SK4o/SK4u can be extended laterally in pairs in the system shown in order to make the intermediate scaffold area more accessible for maintenance and repair purposes. This can shorten maintenance and repair times and consequently enable higher utilization of plant A. A lateral extension in exchange with other units that are only used temporarily, e.g. B. Induction heating or straightening units is also possible.

Due to the rapid cooling spray bars SK1o/SK1u, SK2o/SK2u, SK3o/SK3u of the cooling device arranged between the n-1'th roll stand and the n'th roll stand, a particularly rapid cooling of the hot strip B after the n'th forming pass can be made possible and thus a hot strip B with a particularly fine grain structure of the microstructure can be obtained. This is characterized by improved strength and toughness properties.

Reference symbol list

A

Attachment

b

Hot strip

O

Top of the hot strip

U

Underneath the hot strip

F

Direction of conveyance

n-1

n-1'th roll stand

n

nth rolling stand

L

Distance between the rolling stands n-1 and n

SK1o, ..., SK4o

upper rapid cooling spray bars

SK1u, ..., SK4u

lower rapid cooling spray bars

W

Cooling medium

P

Process computer

M

Measuring device

S

Control device

Claims (10)

1. Plant (A) for producing metallic hot-rolled strip (B),

   with a hot-rolling train for undertaking one or more rolling passes, wherein the hot-rolling train comprises at least two roll stand n-1, n roll stands arranged in succession in conveying direction (F) of the hot-rolled strip (B), and

   with a cooling device,

   wherein the cooling device comprises at least one first upper fast-cooling spray bar (SK1o) for delivery of a cooling medium (W) onto the upper side (O) of the hot-rolled strip (B) between the n-1th roll stand and the nth roll stand, and

   wherein the cooling device comprises at least one first lower fast-cooling spray bar (SK1u) for delivery of a cooling medium (W) onto a lower side (U) of the hot-rolled strip (B) between the n-1th roll stand and the nth roll stand,

   characterised in that

   the cooling device is arranged for covering > 30 percent of the length (L) of the hot-rolled strip (B) between the n-1th roll stand and the nth roll stand with the cooling medium (W); and

   the first upper fast-cooling spray bar (SK1o) and/or the first lower fast-cooling spray bar (SK1u) is or arranged directly behind the outlet of the n-1th roll stand; and

   the nozzle outlet from the fast-cooling spray bars (SK1o, SK1u) has such a configuration that the issuing coolant directly after leaving the outlet-side rolling gap can act on the hot-rolled strip (B) so that the coolant flow acts from a spacing of less than 500 mm, preferably less than 400 mm, particularly preferably less than 350 mm, on the hot-rolled strip (B) and the cooling of the hot-rolled strip (B) commences in terms of space directly after leaving the n-1th roll stand; and

   the rolling train has a tracking system, wherein the tracking system tracks the position of the hot-rolled strip (B) and the control device of the fast-cooling spray bars (SK1o, SK1u) receives a signal for the start as soon as the hot-rolled strip start transits the rolling gap and a time-delayed signal for the stop ensues as soon as the hot-rolled strip (B) has left the rolling gap so that the cooling of the hot-rolled strip (B) commences in terms of space directly after leaving the n-1th roll stand.
2. Plant (A) for producing metallic hot-rolled strip (B) according to claim 1,
   wherein the first upper fast-cooling spray bar (SK1o) and/or the first lower fast-cooling spray bar (SK1u) is or are arranged for the purpose of delivering the cooling medium at a pressure of 1 to 20 bars, particularly 3.0 to 10 bars, more particularly 3.0 to 5 bars.
3. Plant (A) for producing metallic hot-rolled strip (B) according to one of claims 1 and 2,
   wherein the cooling device is arranged for the purpose of cooling down the hot-rolled strip (B) at a specific cooling-down rate of more than 300 Kls per mm of strip thickness, particularly at a cooling-down rate of more than 900 K/s per mm of strip thickness.
4. Plant (A) for producing metallic hot-rolled strip (B) according to any one of the preceding claims,
   wherein the cooling device comprises more than one upper fast-cooling spray bar (SK1o, SK2o, SK3o, SK4o) and/or more than one lower fast-cooling spray bar (SK1u, SK2u, SK3u, SK4u) for delivery of the cooling medium (W) to the upper side (O) or lower side (U) of the hot-rolled strip.
5. Plant (A) for producing metallic hot-rolled strip (B) according to any one of the preceding claims,
   wherein the cooling device comprises at least one upper fast-cooling spray bar (SK4o), which is arranged after the nth roll stand, for delivery of the cooling medium (W) to an upper side (O) of the hot-rolled strip (B), which is constructionally identical to the first upper fast-cooling spray bar (SK1o), and/or at least one lower fast-cooling spray bar (SK4u), which is arranged after the nth roll stand, for delivery of the cooling medium (W) to a lower side (U) of the hot-rolled strip (B), which is constructionally identical to the first lower fast-cooling spray bar (SK1u)
6. Plant (A) for producing metallic hot-rolled strip (B) according to any one of the preceding claims,
   wherein the plant (A) comprises a process computer (P) for determining the quantity of cooling medium and/or distribution of the cooling medium quantity to the fast-cooling spray bar or bars (SK1o, SK1u, SK2o, SK2u, SK3o, SK3u, SK4o, SK4u).
7. Plant (A) for producing metallic hot-rolled strip (B) according to any one of the preceding claims,
   wherein the n-1th roll stand is arranged for undertaking the last rolling pass.
8. Plant (A) for producing metallic hot-rolled strip (B) according to any one of the preceding claims,
   wherein the rolls of the nth roll stand are used in the form of squeeze rollers or drive rollers.
9. Plant (A) for producing metallic hot-rolled strip (B) according to any one of the preceding claims,
   wherein at least one of the upper and lower fast-cooling spray bars (SK1o, SK1u, SK2o, SK2u, SK3o, SK3u, SK4o, SK4u) is arranged to be movable out.
10. Method of producing metallic hot-rolled strip (B) with use of a plant according to any one of the preceding claims 7 to 9,
    wherein the last rolling pass is undertaken with the n-1th roll stand and wherein the hot-rolled strip (B) is cooled-down directly behind the rolling gap of the n-1th roll stand at a specific cooling-down rate of more than 600 K/s per mm of strip thickness.

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