Six-high rolling mill stand and finishing mill train for hot rolling an intermediate strip into a thin strip

Abstract

A six-high rolling (also known as sexto) mill stand that is suited for hot rolling an intermediate strip into a thin strip that is less than 0.8 mm thick. A combined casting and rolling installation that includes the six-high rolling mill stand allowing for hot rolling in long uninterrupted sequences, without any change of the work rolls to obtain a strip with good geometry due to moderate rolling forces.

Claims

1. Six-high rolling mill stand for hot rolling an intermediate strip into a thin strip, the rolling mill stand comprising: an upper work roll and a lower work roll for hot rolling the intermediate strip between the upper work roll and the lower work roll into the thin strip, work roll bending blocks for bending the work rolls in vertical direction; two axial shifting devices for axially shifting the work rolls; two intermediate rolls for supporting the work rolls in vertical direction intermediate roll bending blocks for bending the intermediate rolls in vertical direction; and two backup rolls for supporting the intermediate rolls in vertical direction, wherein each intermediate roll has a first tapered portion, in axial direction followed by an intermediate portion, and followed by a second tapered portion, wherein a roll crown of the intermediate portion follows an even function with respect to the centre of the intermediate portion fin width direction, wherein each tapered portion features a large diameter adjacent to the intermediate portion and a comparatively smaller diameter on outside.

2. Six-high rolling mill stand according to claim 1, wherein each work roll features a cylindrical portion, in axial direction followed by a running surface, and followed by a tapered portion, wherein the upper work roll is arranged in an opposite direction to the lower work roll; and wherein the two axial shifting devices allow the shifting the work rolls in opposite axial directions.

3. Six-high rolling mill stand according to claim 1, comprising at least two work roll cooling devices, wherein each work roll cooling device allows the cooling of multiple, axially spaced cooling zones (Z1 . . . Z12) of the running surface of a work roll with adjustable cooling intensity.

4. Six-high rolling mill stand according to claim 3, wherein the number of multiple, axially spaced cooling zones (Z1 . . . Z12) is at least three.

5. Six-high rolling mill stand according to claim 3, wherein the number of multiple, axially spaced cooling zones (Z1 . . . Z12) is at least five.

6. Six-high rolling mill stand according to claim 3, wherein the number of multiple, axially spaced cooling zones (Z1 . . . Z12) is at least nine.

7. Six-high rolling mill stand according to claim 1, wherein the diameter of a work roll is between 300 and 500 mm, and/or wherein the diameter of an intermediate roll is between 450 and 800 mm.

8. Six-high rolling mill stand according to claim 1, wherein at least two stabilising devices are assigned to each intermediate roll for stabilizing the intermediate roll during rolling in the horizontal and vertical direction.

9. Six-high rolling mill stand according to claim 1, wherein at least two stabilising devices are assigned to each work roll for stabilizing the work roll during rolling in the horizontal and vertical direction.

10. Finishing mill train for hot rolling an intermediate strip into a thin strip, wherein the thickness of the thin strip is <0.8 mm, the finishing mill comprising: two or three four-high mill stands, wherein each four-high mill stand features work roll bending blocks for bending the work rolls of the mill stand in the vertical direction, two or three six-high rolling mill stands according to claim 1, wherein the four-high mill stands are arranged in the transport direction of the strip before the six-high rolling mill stands.

11. Finishing mill train according to claim 10, additionally comprising: a cooling line for cooling the thin strip to coiling temperature, a measurement device for measuring the profile and/or the flatness of the thin strip, wherein the measurement device is arranged in the transport direction of the strip between the last mill stand of the finishing mill train and the first cooling header of the cooling line.

12. Finishing mill according to claim 11, additionally comprising: a controller for controlling the profile and/or flatness of the thin strip, wherein the controller is connected to the measurement device for measuring the profile and/or the flatness of the thin strip, the work roll bending blocks of the four-high mill stands of the finishing mill train, and the work roll bending blocks and the intermediate roll bending blocks of the six-high mill stands of the finishing mill train.

13. Finishing mill according to claim 12, wherein the controller is also connected to the work roll cooling devices for cooling multiple, axially spaced cooling zones (Z1 . . . Z12) of the running surface of a work roll with adjustable cooling intensity.

14. Method for producing a thin strip in a combined casting and rolling installation, wherein the thickness of the thin strip is <0.8 mm, comprising the following steps: continuous casting a steel strand with slab or thin-slab format in a continuous casting machine; liquid-core reduction and/or soft-core reduction of the steel strand in a strand guide of the continuous casting machine, wherein the thickness of the steel strand is reduced by at least 5%; roughing rolling of the reduced steel strand to an intermediate strip in a roughing mill train, wherein the thickness of the intermediate strip is between 8 and 45 mm; finishing rolling the intermediate strip into the thin strip in a finishing mill train, wherein firstly two or three reduction steps are performed subsequently by four-high mill stands, and secondly, two or three reduction steps are performed subsequently by six-high mill stands, wherein the finishing mill train is according to claim 10; measuring a profile and/or a flatness of the thin strip, wherein a measurement device is arranged in the transport direction of the strip between the last mill stand of the finishing mill train and the first cooling header of a cooling line; cooling of the thin strip to coiling temperature in the cooling line; and coiling of the cooled thin strip.

15. Method according to claim 14, wherein the roughing rolling is performed on an uncut steel strand, the finishing rolling is performed on an uncut intermediate strip.

16. Method according to claim 15, wherein the cooling is performed on the uncut thin strip, and the thin strip is cut before the coiling of the cooled thin strip.

17. Method according to claim 14, wherein the steel strand is cut to slabs before roughing rolling, roughing rolling the slabs into intermediate strips, wherein the intermediate strips are joined together before finishing rolling, finishing rolling is performed on the joined intermediate strips.

18. Method according to claim 17, wherein the cooling is performed on the uncut thin strip, and the thin strip is cut before the coiling of the cooled thin strip.

19. Method according to claim 14, wherein the profile and/or the flatness of the thin strip is controlled by a controller taking into account the measured profile and/or flatness of the thin strip by setting the bending of the work rolls of the four-high mill stands of the finishing mill train, the bending of both the work rolls and the intermediate rolls of the six-high mill stands of the finishing mill train, and cooling of multiple, axially spaced cooling zones (Z1 . . . Z12) of the running surface of the work rolls of the six-high mill stands of the finishing mill train with a pre-set cooling intensity.

20. Method for producing a thin strip in a combined casting and rolling installation according to claim 11, additionally comprising heating the intermediate strip to a surface temperature between 90 and 1200 C.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Further advantages and features of the present invention are provided by the following description of non-restrictive exemplary embodiments, wherein the figures show:

(2) FIG. 1 a schematic showing a six-high rolling mill stand according to the invention in a partially cut front view,

(3) FIG. 2 a schematic showing the work, intermediate and backup rolls of the six-high rolling mill stand of FIG. 1,

(4) FIG. 3 a schematic showing the upper work roll, the upper intermediate roll, and the upper backup roll of FIG. 2 separately in greater detail,

(5) FIG. 4 a diagram showing the control range in a 4-high rolling mill stand according to the prior art and a 6-high rolling mill stand according to the invention,

(6) FIGS. 5a and 5b a schematic showing the work roll cooling device for cooling multiple, axially separated cooling zones of the work roll,

(7) FIG. 6 a diagram showing the specific flow rates of the work roll cooling device of FIGS. 5a and 5b in the width direction of the work roll, and

(8) FIG. 7 a schematic showing a combined casting and rolling line having two six-high rolling stands in the finishing rolling train.

DESCRIPTION OF EMBODIMENTS

(9) FIG. 1 shows a schematic front view of a six-high rolling mill stand 1 according to the invention for hot rolling an intermediate strip 2 into a thin strip 3. The intermediate strip 2 is hot rolled between the rolling gap between the upper work roll 4a and the lower work roll 4b. The work rolls 4a, 4b are journaled in roll chocks and may be bent by two work roll bending blocks 8. The work roll bending block 8 on the entry side of the mill stand 1 features two stabilizing devices 16 for locking the work rolls 4a, 4b during hot rolling. The work rolls 4a, 4b are supported by two intermediate rolls 10 in the mill stand. Also the intermediate rolls 10 are journaled in roll chocks and may be bent by four intermediate roll bending blocks 12. The bending forces from the work roll bending blocks 8 and the intermediate roll bending blocks 12 are shown by arrows on the exit side and on the entry side of the mill stand 1, respectively. For reasons of clarity, the bending forces are shown on one side of the mill stand 1 only, although they are present on both sides. The intermediate rolls 10 are supported by two so-called backup rolls 13 in the mill stand. Also the backup rolls 13 are journaled in roll chocks, however, no bending blocks for bending the backup rolls 13 are present. In order to change the rolling gap between the upper and the lower work roll 4a, 4b, one hydraulic cylinder 17 is present to move the roll chocks journaling the backup rolls 13 in the mill stand 1. The pass line level of the work roll 4b is set by the pass line adjusting device 18. As the backup rolls 13 are in contact with the intermediate rolls 10 and the intermediate rolls 10 are in contact with the work rolls 4a, 4b, the rolling gap is changed by the hydraulic cylinder 17. As depicted in FIG. 1, stabilizing devices 16 are present on the exit side of the mill stand 1 between the intermediate roll bending blocks 12 and the roll chocks. It is possible that further stabilizing devices 16 are present on the entry side of the mill stand and between the roll chocks and work roll bending blocks 8.

(10) The work rolls 4a, 4b and the work roll bending blocks 8, the intermediate rolls 10 and the intermediate bending blocks 12, the backup rolls 13 and the axial shifting devices 9 are shown again in FIG. 2 in a side view. It is immediately apparent that each work roll 4a, 4b features a tapered section, followed by a typically ground running surface, and a cylindrical surface. The upper work roll 4a is arranged opposite to the lower work roll 4b. Both work rolls 4a, 4b are connected to a respective separate axial shifting device 9 that allows the shifting of the work rolls 4a, 4b in opposite horizontal directions, i.e. the upper work roll 4a to the right and the lower work roll 4b to the left (see horizontal arrows). The work roll bending blocks 8 and the intermediate bending blocks 12 are shown schematically.

(11) The different portions of the work rolls 4a, 4b, the intermediate rolls 10 and the backup rolls 13 are depicted in FIG. 3. Each work roll 4a, 4b features a tapered portion 7, in axial direction followed by a running surface 6 and a cylindrical portion 5. Each intermediate roll 10 features two tapered portions 7, having a small diameter outside and a comparatively larger diameter further inside, and an intermediate portion 11 in-between. The contour of the intermediate portions 11 follows an even function with respect to the center of the intermediate portion (and even of the intermediate roll 11 itself) in the width direction. The centre of the intermediate portion 11 in width direction is indicated by a dash-dotted line. In this case, the even function is a cylindrical shape of the intermediate portion 11. The bending forces FB1 acting on the upper work roll 4a and the bending forces FB2 acting on the intermediate roll 10 as well as the force FS shifting the work roll 4a in a horizontal, axial direction are depicted by arrows.

(12) In FIG. 4 the control range of the work roll bending blocks 8 and the intermediate bending blocks 12 depicted in FIGS. 1 and 2 of a six-high rolling mill stand according to the invention are compared to the control range of the work roll bending block 8 for a 4-high rolling mill stand. It is evident that the combination of the work roll bending blocks (in the diagram labelled WRB) with the intermediate roll bending blocks (short IRB) results in a much wider control range for both the quadratic component A2 and the quartic component A4. The six-high rolling mill stands 1 according to the invention are therefore superior to 4-high rolling mill stands in terms of geometry control of the rolled thin strip.

(13) FIGS. 5a and 5b depict the upper and lower work rolls 4a, 4b of the six-high rolling mill stand with one work roll cooling device 14 on the entry and three work roll cooling devices 14 on the exit side of the mill stand per work roll, respectively. For sake of conciseness, the intermediate rolls and the backup rolls are not depicted in these FIGS. On the entry side of the mill stand, one work roll cooling device 14 is present per work roll. This work roll cooling device 14 allows the individual cooling of 12 axially spaced cooling zones Z1 . . . Z12 of the running surface 6 of the work roll 4a, 4b (see FIG. 5b). In other words, the cooling intensity of each zone Z1 . . . Z12 may be adjusted independently by a separate valve 15. The valve 15 can either adjust the pressure of the cooling fluid, which consequently changes the flow rate through the spraying nozzle or change the opening time in which the valve 15 is fully open. The change of the opening time also adjusts the total amount of the cooling fluid passing through the spraying nozzle per cycle (see so-called pulse width modulation, short PWM). On the exit side of the mill stand, three work roll cooling devices 14 are present. Each work roll cooling device 14 again features at least one valve 15 for setting the effective flow rate through the cooling nozzle.

(14) With respect to FIG. 5b it is noted that according to a simple first embodiment of the invention, it is in most cases sufficient that the cooling zones cover the running surface 6 of the work rolls 4a, 4b only, and possibly some overlap with the tapered portion and/or the cylindrical portion of the work rolls exists. Therefore, e.g. only the cooling zones Z3 . . . Z12 depicted in FIG. 5b are switched on. In this case, the cooling devices 14 remain stationary even though the work rolls 4a, 4b may be shifted in an axial direction. According to an alternative embodiment of the invention, the cooling devices 14 do not remain stationary and are shifted axially synchronously with the respective work rolls. The shifting of the cooling devices 14 can take place by either the long stroke axial shifting devices shifting the work rolls, or by separate axial shifting devices. In either case, it is advantageous that the cooling zones cover both the tapered portion 7 and the running surface 6 of the work rolls. At the beginning of a rolling sequence, only the cooling zone Z3 . . . Z12 are switched on and the valves 15 for the cooling zones Z1 and Z2 are closed. Towards the end of the rolling campaign and as the upper work roll 4a is shifted to the right during rolling, the zones Z1 . . . Z10 are switched on and the valves 15 for zones Z11 and Z12 are shut off. By doing so, the regions of the work rolls contacting the strip are cooled at all times.

(15) FIG. 6 shows the specific flow rates of the three work roll cooling devices 14 located on the exit side of the mill stand of FIG. 5. One of the work roll cooling devices 14 supplies the basic cooling of the work roll, amounting to 30% of the max. total cooling intensity. The basic cooling is almost constant across the width of the work roll, falling to some 70% at the edges. The additional cooling #1 amounts to 35% of the max. total cooling intensity and follows a cosine function with its peak at the centre of the barrel. The additional cooling #2 also amounts to 35% of the max. total cooling intensity and follows a negative cosine function with its minimum at the centre of the barrel. The frequency of the additional cooling function #2 is twice the frequency of the additional cooling function #1. Assuming that all three work roll cooling devices 14 operate at maximum flow, the max. cooling intensity is at the centre of the barrel (flow rate almost 4) compared to a flow rate of 1.5 at the edges of the barrel.

(16) The six-high rolling mill stand according to the invention is particularly advantageous for finishing rolling high quality ultra-thin steel strip with a final thickness after the last roll stand <0.8 mm, preferably s 0.6 mm. The diameter of the work rolls is typically between 300 and 500 mm and consequently considerably smaller than the diameter of work rolls in four-high mill stands. The smaller diameter results in a considerably smaller rolling force at the same ratio of thickness reduction. Due to the reduced rolling force, the geometrical properties, such as profile and/or flatness, of the thin steel strip are greatly improved. The application of six-high rolling mill stands is particularly advantageous as the third, fourth and/or fifth mill stand in a finishing mill train of a combined casting and hot rolling installation, where thin (or ultra-thin) steel strip is produced that may serve as substitute material for cold rolled steel strip.

(17) FIG. 7 shows a combined casting and hot rolling installation 40 featuring a continuous casting machine 41 with a bow-type strand guide 42, in which the strand coming from the mould is subjected to a liquid-core and/or a soft-core reduction. By doing so, the strand having a thickness of 110 mm immediately after the mould is reduced to a thickness of 100 mm at the end of the bow-type strand guide 42. The reduced strand is then rough rolled in a roughing mill train 43 to a so-called intermediate strip having a thickness of 16 mm. The temperature of the intermediate strip can be adjusted by a heater 44. After descaling the heated intermediate strip, the intermediate strip is finish rolled in a finishing mill train 20. The first to third rolling step in the finishing mill train 20 are done by 4-high mill stands 21, the fourth and the fifth rolling step are done by six-high rolling mill stands 1, respectively. The end thickness of the ultra-thin steel strip is 0.6 mm. All rolling steps both in the roughing mill train 43 and in the finishing mill train 20 are performed on the uncut strand/strip, i.e. in endless mode. After the finishing rolling, the profile and/or the flatness of the ultra-thin steel strip is measured by a measurement device 23. After that, the temperature of the steel strip is reduced to coiling temperature in the cooling line 22, the endless strip is cut to length/weight by shears and coiled by at least two coilers. In order to produce ultra-thin strip with top notch geometrical properties, the measurement device 23 continuously measures both the profile and the flatness of the thin steel strip and feeds the measurement data into a controller 30. As depicted, the controller 30 compares the measured profile of the thin strip PR.sub.is to the reference value for the thin strip PR.sub.Ref and creates control values for the work roll bending blocks of the 4-high rolling mill stands 21, the work roll bending blocks of the 6-high rolling mill stands 1 and the intermediate roll bending blocks of the 6-high rolling mill stands 1. By doing so, the geometric properties of the ultra-thin steel strip are equally good or at least almost comparable to cold rolled steel strip produced by state-of-the-art technologies.

(18) Although the invention has been illustrated more specifically and described in detail by the preferred exemplary embodiments, the invention is not restricted by the examples disclosed and other variations can be derived therefrom by a person skilled in the art without departing from the scope of protection of the invention.

REFERENCE SIGNS LIST

(19) 1 Six-high rolling mill stand 2 Intermediate strip 3 Thin strip 4a Upper work roll 4b Lower work roll 5 Cylindrical portion 6 Running surface 7 Tapered portion 8 Work roll bending block 9 Axial shifting device 10 Intermediate roll 11 Intermediate portion 12 Intermediate roll bending block 13 Backup roll 14 Work roll cooling device 15 Valve 16 Stabilising device 17 Hydraulic cylinder 18 Pass line adjusting device 20 Finishing mill train 21 Four-high rolling mill stand 22 Cooling line 23 Measurement device 30 Controller 40 Combined casting and rolling installation 41 Continuous casting machine 42 Strand guide 43 Roughing mill train 44 Heater FB1, FB2 Bending force FS Displacement force PR.sub.is Measured profile of the thin strip PR.sub.Ref Reference value of the profile of the thin strip Z1 . . . Z12 Axially spaced cooling zone