Patent Application
20240359224
The invention concerns a six-high rolling mill stand (also known as sexto rolling mill stand) for hot rolling an intermediate strip into a thin strip and a method for producing a thin strip in a combined casting and rolling installation, wherein the thickness of the thin strip is <0.8 mm.
According to a first aspect of the invention, the invention concerns a 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.
According to a second aspect of the invention, the invention concerns a 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 the 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; 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 a cooling line; cooling of the thin strip to coiling temperature in the cooling line; and coiling of the cooled thin strip.
WO2017/215595 discloses a four-high rolling mill stand (also known as quarto 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, 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 two axial shifting devices for shifting the work rolls in opposite axial directions. In addition, the document discloses a method for producing a thin strip in a combined casting and rolling installation, comprising the following steps: continuous casting a steel strand with slab or thin-slab format in a continuous casting machine; roughing rolling of the reduced steel strand to an intermediate strip in a roughing mill train; finishing rolling the intermediate strip into the thin strip in a finishing mill train; cooling of the thin strip to coiling temperature in a cooling line; and coiling of the cooled thin strip. Although the rolling mill of WO2017/215595 is suitable for hot rolling long uninterrupted sequences without any change of the work rolls, the rolling forces become very high when producing ultra-thin strip with a thickness<0.8 mm.
EP3595822 discloses a 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; two intermediate rolls for supporting the work rolls in the vertical direction; and two backup rolls for supporting the intermediate rolls in the vertical direction. In addition, the document discloses a 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; 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 40 mm; finishing rolling the intermediate strip into the thin strip in a finishing mill train; cooling of the thin strip to coiling temperature in the cooling line; and coiling of the cooled thin strip. Although the rolling mill of EP3595822 is suitable for producing ultra-thin strip with a thickness<0.8 mm, the solution is not suitable for hot rolling long uninterrupted sequences without any change of the work rolls.
U.S. Pat. No. 5,622,073 discloses a six-high rolling mill stand for hot rolling an intermediate strip into a strip, the rolling mill stand comprising:
It is noted that the S shaped roll crown of the intermediate rolls follows an odd function with respect to the centre of the intermediate roll in the width direction. In other words, the contour of the rolls is asymmetric with respect to the center of the rolls. During rolling, the intermediate rolls are axially shifted in order to control the profile of the strip.
WO2018/167711 discloses a method for producing thin strip in a combined casting and rolling installation, wherein the thickness of the thin strip is <0.8 mm, comprising the following steps:
Although six-high rolling mill stands are known in the prior art, the known mill stands are either not suitable for hot rolling long uninterrupted sequences, particularly in endless mode, without any change of the work rolls, or the mill stands are not optimally suited for producing ultra-thin strip with a thickness<0.8 mm.
It is the primary object of the invention to come up with a six-high rolling mill stand that is suitable for hot-rolling of ultra-thin steel strip with a thickness<0.8 mm, say 0.6 mm and below, in a combined casting and rolling installation, whereby the hot rolling can be done in long uninterrupted sequences, particularly in endless mode, without any change of the work rolls, and very good geometry of the thin strip due to moderate rolling forces.
Another object of the invention is to come up with a method for producing an ultra-thin steel strip in a combined casting and rolling installation with low production costs and very good geometry.
The first object of the invention is solved by a six-high rolling mill stand for hot rolling an intermediate strip into a thin strip, preferably having a thickness<0.8 mm, according to claim 1, the rolling mill stand comprising:
Due to the six-high arrangement of the rolling mill stand, the running surface of the work rolls may have a small diameter, e.g. between 300 and 500 mm, resulting in low rolling forces and a good geometry of the resulting thin strip. The geometry, i.e. the profile and flatness, of the thin strip can be adjusted by both the work roll bending blocks for bending the work rolls in vertical direction and the intermediate roll bending blocks for bending the intermediate rolls in vertical direction. The combination of both bending the work rolls and bending the intermediate rolls results in a broad two-dimensional field that allows the effective adjustment of profile and flatness of the rolled ultrathin strip in a broad manner.
Compared to six-high rolling mill stands from the prior art, bending blocks for both the work rolls as well as for the two intermediate rolls are present. Typically, the upper work roll is bent by the work roll bending block in the upward vertical direction, whereas the lower work roll is bent by the work roll bending block in the downward vertical direction. The intermediate rolls may be bent in analogous directions by the intermediate roll bending blocks. Another significant difference to six-high rolling mill stands from the prior art is that each of the two intermediate rolls features a first tapered portion, in axial direction followed by an intermediate portion, and in axial direction followed by a second tapered portion. This in retrospect relatively simple modification of the intermediate roll's shape reduces the Hertz’ stresses between the end regions of the running surface of the work rolls and the corresponding regions of the intermediate rolls significantly. As the edge regions in the width direction of the thin strip are most strongly affected by work roll wear, the special shape of the intermediate rolls reduces the work roll wear considerably.
The six-high rolling mill stand according to the invention can either feature
The earlier mentioned embodiment has been known from conventional hot strip mills for a long time. The cyclical shifting of the work rolls aims to distribute the wear across the running surface of the work rolls.
The latter mentioned embodiment allows even longer uninterrupted rolling sequences due to the special shape of the work rolls, each roll having a cylindrical portion, a typically ground running surface, and a tapered portion, the opposite arrangement of the work rolls in the mill stand, and two long-stroke axial shifting devices for shifting the work rolls in opposite axial directions. The shifting of the work rolls in opposite axial directions (see WO 2017/215595) ensures that the wear of the work rolls has no or at least very little effect on the geometry of the thin strip. Such a six-high rolling mill stand is ideally suited for hot-rolling long uninterrupted sequences in a combined casting and rolling installation, e.g. an Arvedi ESP line.
According to the invention, each intermediate roll features
Each tapered portion features a large diameter adjacent to the intermediate portion (i.e. inside) and a comparatively smaller diameter on the outside of the tapered portion.
Contrary to the teaching of U.S. Pat. No. 5,622,073, the roll crown of the intermediate rolls according to the invention follows an even function, not an odd function. By doing so, the roll crown is symmetric with respect to center of the intermediate portion (or even the roll) in width direction. By having intermediate rolls with a symmetric roll crown, the deformation of the intermediate rolls due to rolling force and thermal deflection can be compensated by the intermediate roll bending blocks allowing the being of the intermediate rolls in vertical direction. The simplest contour of an intermediate rolls where the roll crown follows an even function is a cylindrical shape. However, other even shapes, such as a constant function y=A with A∈, a parabola y=A·x2 or more generally following the function y=A·x2n with n∈N, or following a cosine function y=A·cos(x) . . . are possible.
In a preferred embodiment of the invention, the six-high rolling mill stand features two work roll cooling devices, one assigned to the upper work roll and the other assigned to the lower work roll, for the cooling of multiple, axially spaced cooling zones of the running surface of the work roll with adjustable cooling intensity. By doing so, the geometry, i.e. the profile and/or flatness, of the thin strip can be adjusted not only by bending the work rolls and the intermediate rolls, but also by cooling the running surface of the work rolls with adjustable cooling intensity. This allows an effective control of the geometry of the thin strip.
At least one, preferably two, work roll cooling device is present on the entry side of the rolling mill and at least one, preferably two or three, work roll cooling device is present on the exit side of the rolling mill.
The work roll cooling devices may either be shifted synchronously with the axial shifting of the work rolls or remain stationary in the axial direction. Particularly in the former case, it may be advantageous to use valves to switch off cooling zones which are no longer in contact with the strip.
According to a preferred embodiment, the number of multiple, axially spaced cooling zones is at least three, preferably at least five. For effectively controlling the geometry of the rolled strip, the number of cooling zones can be increased further, e.g. up to 29.
It is preferred to use work rolls having a diameter between 300 and 500 mm, and/or to use intermediate rolls having a diameter between 450 and 800 mm. The diameter of the work rolls directly influences the rolling force during hot rolling, wherein a small diameter results in a small rolling force and vice versa.
In order to “lock” the work-rolls and/or intermediate rolls in the vertical and horizontal direction, it is preferred that at least two stabilising devices are assigned to each work roll and/or intermediate roll. The locking can be done prior to the commencement of a rolling sequence, wherein the sequence can be conducted in endless, semi-endless or even batch rolling mode. By locking the rolls, shocks—e.g. a rapidly increasing rolling force at the beginning of the rolling sequence, do not result in a corresponding displacement of the roll in vertical and/or horizontal direction. In any case, such displacement will be a lot smaller.
A finishing mill train according to the invention comprises: two or three, preferably 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 vertical direction; two or three, preferably two, six-high rolling mill stands according to the invention, wherein the four-high mill stands are arranged in the transport direction of the strip before the six-high rolling mill stands.
As four-high mill stands are significantly simpler and cheaper than six-high rolling mill stands, it makes sense to use four-high mill stands at the beginning of the finishing mill train and to use six-high mill stands towards the end of the finishing mill train. In a preferred embodiment, all four-high mill stands are comprised in a first group and all six-high mill stands are comprised in a second group, wherein the 1st group is arranged ahead of the 2nd group.
In a more preferred embodiment of the invention, between the last mill stand of the finishing mill train and the first cooling header of the cooling line a measurement device for measuring the profile and/or the flatness of the thin strip is arranged. The measurement device allows quality control of the rolled thin strip and—as may be seen below—can be used to feed measurement data into a controller in order to control the geometry of the thin strip in the finishing mill train.
According to an even more preferred embodiment, the finishing mill train features 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, the work roll bending blocks and the intermediate roll bending blocks of the six-high mill stands of the finishing mill train, and preferably the work roll cooling devices for cooling multiple, axially spaced cooling zones of the running surface of a work roll with adjustable cooling intensity.
The second object of the invention is solved by a 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:
In the combined casting and rolling installation, a continuous casting machine continuously casts liquid steel into a steel strand with slab or thin-slab format. By reducing the thickness of the steel strand by a liquid-core and/or soft-core reduction in the strand guide of the casting machine, the metallurgical quality of the steel strand is improved, the thickness is reduced and the speed of the steel strand is increased. The so-called liquid-core reduction reduces the thickness of the strand when the centre of the strand is still liquid; likewise the so-called soft-core reduction reduces the thickness of the strand when the centre of the strand is still mushy, i.e. no longer liquid and not yet fully solidified. After the liquid-core and/or soft-core reduction, the thickness of the strand is reduced further by roughing rolling to an intermediate strip with a thickness between 8 and 45 mm. The roughing rolling is typically done by two, three or four roughing stands. In order to reach a specific, e.g. austenite or ferrite, grain in the thin strip, the last reduction step in the finishing mill train needs to be performed at a specific temperature. Depending on the length of the combined casting and rolling installation, the casting speed etc. the surface temperature of the intermediate strip may be heated to a temperature between 900 and 1200° C. The heating may be done by induction heating preferably. In any case, the intermediate strip is finish rolled into the thin strip in a finishing mill train, wherein the first two or three, preferably three, reduction steps are performed subsequently by four-high mill stands, and after this, two or three, preferably two, reduction steps are performed subsequently by six-high mill stands. The six-high mill stands is preferably according to one of the claims 8 to 10. After hot rolling and before cooling the thin strip to coiling temperature, the profile and/or the flatness of the thin strip is measured by a measurement device. Finally, the cooled thin strip is coiled.
According to a first preferred embodiment, the roughing rolling is performed on the uncut steel strand, the finishing rolling is performed on the uncut intermediate strip, preferably the cooling is performed on the uncut thin strip, and the thin strip is cut before the coiling of the cooled thin strip.
This embodiment is particularly suitable for traditional combined casting and rolling installations, wherein the continuous casting machine, the roughing mill train, the finishing mill train, the cooling line and the coilers are aligned in-line. The endless operation of the casting and rolling installation allows the production of ultra-thin steel strip without the risk of cobbles in the line due to strip heads moving with high speed and getting caught by auxiliary devices. Due to the endless operation, no strip heads are present and in the steel strand, the intermediate strip and the thin strip some tension is present, such that a “flying strip head” cannot occur.
According to a second preferred embodiment, the steel strand is cut to slabs before roughing rolling, the slabs are roughing rolled into intermediate strips, the intermediate strips are joined together before finishing rolling, the finishing rolling is performed on the joined intermediate strips, preferably the cooling is performed on the uncut thin strip, and the thin strip is cut before the coiling of the cooled thin strip.
This embodiment allows the production of 4 million metric tons/year on a single casting and rolling installation having more than 1 continuous casting machine. Also in this case, the finishing rolling and the cooling take place in an uncut fashion, whereby the risk of a “flying strip head” is greatly reduced.
In another preferred embodiment of the invention, 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
Further advantages and features of the present invention are provided by the following description of non-restrictive exemplary embodiments, wherein the figures show:
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
The different portions of the work rolls 4a, 4b, the intermediate rolls 10 and the backup rolls 13 are depicted in
In
With respect to
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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21187459.9 | Jul 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/070546 | 7/21/2022 | WO |
