ROLLING METHOD

Abstract

A method of rolling a metal plate from an ingot or thick slab (1,2) includes setting a work roll gap with a mechanical screw (24) and rolling the ingot or slab through a first pass to produce a rolled product (21). The rolled product (21) is removed from the roll gap and the mechanical screw (24) is used to set a reduced roll gap (34a). The rolled product (21) is rolled through the reduced roll gap (34a) over a partial pass extending over less than the full length of the rolled product (21), to form a further rolled product (22, 21); and the further rolled product is removed from the roll gap. Repeat prior step. Then turn the rolled plate and carrying out a further roll pass in a width direction of the plate.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation in part of PCT/EP2014/067678, filed Aug. 19, 2014, which claims priority of Great Britain Patent Application No. 1316917.2, filed Sep. 24, 2013, the contents of which are incorporated by reference herein. The PCT International Application was published in the English language.

TECHNICAL FIELD

This invention relates to a method of rolling metal, in particular for production of high quality thick plate from ingots or thick slabs.

TECHNICAL BACKGROUND

Market demand for high quality thick plate for the construction industry requires that the plates are rolled from either traditional ingots or thick cast slabs. Both create significant processing problems and yield loss on the final plate. Normally, ingots have variations or tapers in both thickness and width down their length, which have to be removed during rolling. Once the variations have been removed, the ingot can be processed in the same manner as a thick cast slab. For this application, reference to either ingot or thick slab should include the other, unless otherwise stated.

Traditionally thick plate rolling from ingots has been done using a rolling mill and a detached edger in a series of reversing passes, for example as described in JP01053703.

Starting from a tapered slab, JP58044904 describes the use of tapered rolling to spread the material in the tapered slab, then turning the rolled material and applying further rolling, for eventually forming a rectangular plate.

As described in a paper given at the 49th Rolling Seminar—Processes, Rolled and Coated Products, Vila Velha, Brasil, October 2012, entitled—Production of high quality thick construction plate from ingots and thick slabs, by S Samanta et al, mathematical models can be used with high speed long stroke hydraulic gap control cylinders to remove thickness and width variations before standard thick cast slab processing of the plate to minimize poor edge shape and increase final yield.

However, a number of older mills are either not suitable or economical to convert to hydraulic gap control, thereby limiting the type of plate that they can produce.

DESCRIPTION OF THE INVENTION

In accordance with the present invention, a method awning a metal plate from an ingot or thick slab comprises in sequence: setting a work roll gap with a mechanical screw and rolling the ingot or slab through a first pass through the roll gap to produce a rolled product; removing the rolled product from the roll gap; using the mechanical screw to set a reduced roll gap; rolling the rolled product through the reduced roll gap over a partial pass, wherein the partial pass extends over less than the full length of the rolled product, to form a further rolled product; and removing the further rolled product from the roll gap. There may be a series of successive stages of rolling the rolled product through a further reduced roll gap over a further partial pass, wherein each successive partial pass extends over less than the length of the preceding partial pass, forming a stepped profile of the rolled product. After the series of rolling stages, the method further comprises turning the rolled plate and carrying out a further roll pass in a width direction of the plate, herein called broadsiding.

Rolling a metal plate, whether from an ingot or thick slab, using the method of the present invention to produce a rolled plate having a stepped profile allows older screwdown mills to be used to roll plate which has the required quality, without the loss of yield, rendering the process uneconomical. Broadsiding of the rolled plate converts the stepped thickness profile to a flattened thickness profile and the narrow width region to be widened, thereby causing the plate rolled to become a flattened rectangle.

Preferably, the method further comprises using the mechanical screw to seta further reduced roll gap in each of the successive stages described above; rolling the further rolled product through the further reduced roll gap over a partial pass, in each stage with the partial pass extending over less than the full length of the further rolled product.

Preferably, the method further comprises repeating the steps of removing the further rolled product from the roll gap, using the mechanical screw to set a further reduced roll gap and rolling the product over a partial pass for a set number of iterations to produce a rolled plate. Preferably, the number of iterations is determined according to parameters of required yield loss and rolling time. For each iteration, a section of the rolled product furthest from the work rolls, is left unrolled.

Preferably, the method further comprises counting the number of revolutions of the roll as the rolled product is removed from the roll gap to allow the next roll gap to be set; determining a difference in thickness between adjacent roll gap thickness settings; using the number of revolutions and determined difference in thickness to calculate the length of the product; and thereby deriving the length of the rolled product to be rolled at the next rolling stage.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of a method of rolling a metal plate from an ingot or thick slab in accordance with the present invention will now be described with reference to the accompanying drawings in which:

FIG. 1A shows a top view before rolling of an ingot having a width taper across the ingot in the prior art;

FIG. 1B shows a front view of the ingot having a thickness taper in the prior art;

The arrow indicates conventional rolling.

FIG. 1C shows a top view of the ingot after conventional rolling, and the increased width taper in the prior art;

FIG. 1D shows a front view of the ingot after conventional rolling and no thickness taper in the prior art;

FIG. 2A shows a top view of an ingot before rolling in the prior art and;

FIG. 2B shows a front view of the ingot before rolling in the prior art;

FIGS. 3A, 3B and 3C illustrate how the resulting ingot of FIGS. 2A and 2B can be rolled in the prior art to form a rectangular product, wherein the rolled ingot of FIGS. 2A and 2B has been rotated by 90°;

The arrow indicates a conventional rolling;

FIG. 3C indicates a plate produced by conventional rolling and being without taper in the prior art;

FIGS. 4A to 4D illustrate an example of steps in a method according to the present invention;

FIG. 5A is a perspective view of an ingot that has passed through the rolling process of FIGS. 4A to 4D several iterations, before a broadsiding step;

FIG. 5B is a front view of that ingot;

FIG. 5C is a top view thereof;

FIG. 5D is a front view of a plate formed from the ingot of FIG. 5B after a broadsiding step; and

FIG. 5E is a top view of the plate after the broadsiding step.

DESCRIPTION OF A PRIOR ART EMBODIMENT

Use of edger controlled multiple reversing passes to produce thick plate has been the most common method to date, although the advent of hydraulically operated automatic gauge control has enabled mills to be constructed which are able to overcome the problems of rolling tapered ingots or thick slabs, while still providing sufficient austenite strain for a fine-grained, high quality product. However, there are still many older mills using mechanical screw roll loading technology which either are not suitable or not economical to adapt to hydraulic cylinders and automatic gauge control.

The present invention aims to improve the yield during thick plate production in these screwdown mills.

FIGS. 1A-1D illustrate how traditional cast ingots are tapered in thickness and width along their length. The ingot 1, in top view FIG. 1A, has a width taper, i.e. it tapers across its width direction from lateral side to side in FIG. 1A, becoming narrower from side 12 to side 12A in width as it becomes thicker from bottom 13 to top 14. If standard hydraulically controlled rolling loads are applied to this type of ingot, without any special rolling strategy, then it can be seen, after rolling for example, from FIG. 1A to FIG. 1C that the width taper increases into FIG. 1C, wherein the ingot is changed in its form to ingot 3, and broken lines enclose regions 5, which indicate the increase in width taper.

At the same time, conventional rolling of the ingot 1 of FIG. 1B causes the resulting ingot 3 to have uniform thickness, and the thickness taper of ingot 1 in FIG. 1B disappears as the thickness taper is reduced from FIG. 1B to FIG. 1D. The result is a plate 4 with uniform thickness as in FIG. 1D, but with a width that tapers down along the length of the ingot as in FIG. 1C. This results in a large amount of yield loss when shearing to form a rectangular product for sale.

Using advances in hydraulic control of rolling loads, a process has been developed to add a variable thickness taper to the ingot, which is inversely proportional to the ingot width taper, as illustrated by FIGS. 2A and 2B. The width of ingot 6 decreases in FIG. 2A, while the thickness of that ingot decreases in FIG. 2B. The dashed lines and the areas 8, 9 that the lines enclose define the tapers. The respective tapers 9 showing width and 8 showing thickness are inverse tapers, that is one taper increases as the other decreases.

This taper can then be rolled out by turning the ingot through 90° rotation, as shown in FIGS. 3A and 3B, and rolling it again in the width direction (called broadsiding) in order to spread material and form a rectangular product, as shown in FIG. 3C. That plate 10 has no width or thickness taper. It has been assumed that these advantages are only achievable where automatic gauge control and hydraulic cylinders are installed. The present invention provides a method by which similar improvements can be achieved in older screwdown mills.

DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

FIGS. 4A to 4D illustrate an example of the method of the present invention for rolling tapered ingots or thick slabs using a screwdown system for causing multiple roll gap changes in a rolling mill 24. An ingot having both a width taper and a thickness taper, as in FIGS. 1A and 1B, is transformed into an ingot having a stepped profile, schematically illustrated in FIGS. 5A, 5B and 5C. In FIG. 4A, the ingot or slab 20 is supported on a roller table 27 and is moved in the direction of the arrow 30 to enter a roll gap 34a between opposing work rolls 25 which roll gap has been pre-set. The slab 20 is rolled to a first thickness, as shown in FIGS. 4A and 4B to produce rolled slab 21 in FIG. 4B, and the entire rolled slab 21 exits the work roll gap supported on roller table 26. Mechanical screws in plate mills cannot usually be moved under load, so as shown in FIG. 4B, at least one screw 24 is operated to reduce the gap to 34b between the work rolls 25 in FIG. 4C for the next pass of rolled plate 21, typically narrowing the gap to 34b by moving the at least one work roll 25 in the direction of the arrow 31.

In the next pass, shown in FIG. 4C, the rolled plate 21 is moved through the now reduced roll gap 34b in the opposite direction of the arrow 32, but only a part 22 of the length of the rolled plate 21 is rolled again in gap 34b. When the required length of the part of the rolled plate has been rolled to the new thickness 34b, the rolling is stopped and. the plate is reversed out of the mill to a position as in FIG. 4D. The plate is now formed of two parts 22, 21 of different thickness, as illustrated in FIG. 4D. The at least one screw 24 is operated to change the roll gap by movement of the roll 25 in the direction of the arrow 33 to the next required smaller roll gap 34c. Then the process of rolling part of the length of part 22, stopping and reversing out of the roll gap is repeated as in FIGS. 4C and 4D. For each rolling process, the work rolls 25 are set to have a slightly smaller gap than the previous roll gap and the plate is then rolled again. These are successive iterations of the rolling process. See FIGS. 5A and 5B.

For each iteration, part of the previously rolled section is not rolled again, but the subsequent roll pass finishes at a predefined boundary 35 between the previously rolled thickness, e.g. 34a and the new thickness, e.g. 34b. Thus, the slab formed has a section, e.g. 21 of the thickness of the first roil gap and a respective section of the thickness, e.g. 22, of each subsequent roll gap. For each change in roll gap, the slab is reversed out of the work roll gap, so that the gap can be adjusted using the mechanical screws. Then a next rolling pass reduces the thickness of the slab over a partial length, but does not roll all of the length of the previously rolled sections again. See FIGS. 4C and 4D. In each successive pass, the rolling is not over the full length, but stops at a boundary, e.g. 35 between the immediately preceding section and the most recent section. Iterations of rolling, reversing out and adjustment of the roll gap continue until a desired minimum thickness of the final section 37 of the ingot has been reached. Several iterations successively reduce the thickness of the ingot being rolled in steps. See the resultant ingot of FIGS. 5A and 5B.

In each rolling pass, the rolled product becomes longer in the advancing direction, so in order to control the point to which each subsequent pass should roll, the number of revolutions of the roll are counted. The difference in thickness between each roiled step 21, 22, 37 along the taper is known from the different gaps 34a, b, c, etc. produced by each different screw setting, allowing a calculation of how much longer the slab has become and therefore how far to go back in for the next rolling stage. Instead of a constant ramp change, which is used in systems having an AGC cylinder controlled system, in a screwdown mill, multiple step changes are induced to approximate the desired constant ramp change during the introduction of variable thickness taper. (See FIGS. 5A and 5B) This is performed by adjusting the roll gap using the mechanical screw for multiple rolling stages of decreasing the roll gap and increasing the roll length until the entire length is rolled. The result produced is transformed into a plate with a stepped thickness profile similar to a staircase. (See FIGS. 5A and 5B.) The resulting profile is illustrated in FIGS. 58 showing the front view and 5C, which shows the profile in top or plan view from above at 38 in FIG. 5C. and in front view at 39 in FIG. 5B.

After all of the roll passes in the long direction, the ingot 11, is rotated 90° on its roll plate on which the ingot is supported. Then another roll pass is made, now in the width direction of the rotated ingot, a step called broadsiding. This further roll pass transforms the ingot into a rectangular plate 40, seen in front view in FIG. 5D and in top view in FIG. 5E. In those Figures, both the previously thinner part 37 and the previously thicker parts 21 and 22 et al are further reduced in thickness to the thickness of plate 14 and the reduced thickness cooperates with the tapered width of the ingot to form the plate 14 as a rectangle, as in FIG. 5E. The broadsiding reduces the thickness of all parts 21, 22 . . . 37 to the thickness of the rectangular plate.

The precise number of step changes used in the rolling method is determined according to the process requirements. Where yield loss is less of an issue, a high yield loss is accepted by using fewer steps to get a low rolling time per ingot. If rolling time is not an issue, but reducing yield loss is important, then a greater number of steps are used, over a longer period of time.

As with the hydraulic cylinder automatic gauge control system of modern mills, the method applied to the screwdown mill does not require the use of an edger with rolls before or after the mill to impart force to the edges on the plate. This helps make the process simpler and applicable to using basic mill technology.

Although, a mechanical method of this type takes longer than using single pass AGC cylinder loading and results in more yield loss due to the spreading of plate steps into a saw tooth profile edge in final pass, the result is an improvement on existing operation of screwdown mills which can process material of the required quality.

The present invention provides a process for roiling steel ingots, with both width and thickness tapers, into plate. The process may be used where the resulting plate has a thickness above 120 mm, giving more uniform thickness and width throughout, without the need to use an edger in any passes. Mechanical screw loading using multiple unfinished passes, with discrete roll gap change between each, forms a stepped thickness profile. A further pass in width direction (broad siding) is used to convert the thickness profile to a width increase in the plate geometry, as shown at 14 in FIG. 5E.

Claims

1. A method of rolling a metal plate from an ingot or thick slab in a screwdown mill featuring rolls defining a roll gap between the rolls and a mechanical roll gap screw, the method comprising:

setting a first work roll gap between the rolls defining the roll gap, using the mechanical roll gap adjusting screw, rolling the ingot or slab through a first pass extending through the first roll gap over the full length of the ingot or slab in the rolling direction to produce a first rolled product;

removing the first rolled product from the first roll gap after the first pass;

operating the first roll gap mechanical screw to set a second, reduced work roll gap reduced from the first roll gap;

performing a first rolling iteration comprising rolling the first rolled product through the second, reduced roll gap over a partial pass, the partial pass extending over less than the full length of the first rolled product in the rolling direction, to form a second rolled product;

removing the second rolled product from the roll gap at conclusion of the first iteration;

then turning the second rolled product through 90° and performing a further roll pass on the second rolled product in a width direction broadsiding the second rolling product to form a further rolled product.

2. A method according to claim 1, further comprising:

after removing the further rolled product from the roll gap and prior to the turning of the further rolled product, operating the roll gap mechanical screw to set a further third reduced roll gap; and

rolling the still further rolled product through the further third reduced roll gap over a partial pass, the partial pass extending in the rolling direction over less in the rolling direction than the full length of the further rolled product.

3. A method according to claim 1, further comprising:

successively repeating the steps each time of removing the then further rolled product from the roll gap, operating the mechanical screw to successively set each time a further reduced roll gap, and rolling the then further product over a successive partial pass; and

repeating the steps for a set number of iterations to produce a rolled plate.

4. A method according to claim 3, further comprising selecting a number of the iterations according to parameters of required yield loss and rolling time.

5. A method according to claim 1, further comprising counting a number of revolutions of the roll as the rolled product is removed from the roll gap for setting the next roll gap for obtaining a difference in thickness between adjacent roll gap thickness settings; using the number of revolutions and the difference in thickness for calculating the then length of the rolled product for thereby deriving the length of the rolled product to be rolled at the next rolling stage.

6. A method according to claim 3, wherein each partial pass through a successively further reduced roll gap is of a length over the product less than the length over the product over the length of the respective partial pass over the preceding roll product that had passed the preceding roll gap.

7. A method according to claim 6, wherein the lengths of the successive partial passes are selected to define a stepped surface of the product which has passed through a respective successive roll gap in each partial pass.