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

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 slabs. For the purpose of this application reference to either ingot, or thick slab should be read as including 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, so 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 minimise 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, so limiting the type of plate that they can produce.

In accordance with the present invention, a method of rolling a metal plate from an ingot or thick slab comprises setting a work roll gap with a mechanical screw and rolling the ingot or slab through a first pass to produce a rolled product; removing the rolled product from the roll gap and using the mechanical screw to set a reduced roll gap; rolling the rolled product through the reduced roll gap over a partial pass, the partial pass extending 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; wherein the method further comprises turning the rolled plate and carrying out a further roll pass in a width direction of the plate..

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 thickness profile to a width increase in the plate geometry.

Preferably, the method further comprises using the mechanical screw to set a further reduced roll gap; rolling the further rolled product through the further reduced roll gap over a partial pass, 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.

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:

Figure 1 illustrates how conventional rolling of ingots tapered in thickness and width down their length increases the width taper, as thickness reduces;

Figure 2 illustrates how the use of hydraulic control of rolling loads can be used to apply a thickness taper inversely proportional to the width taper in an ingot or thick slab; Figure 3 illustrates how the resulting slab of Fig.2 can be rolled to form a rectangular product.

Figures 4a to 4d illustrate an example of a method according to the present invention; and,

Figures 5a and 5b illustrate, in plan and cross-section, the changes as the method of Fig.4 converts a tapered product to a rectangular product.

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, whilst 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 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.

Fig.1 illustrates how traditional cast ingots are tapered in thickness and width down their length. If standard rolling is applied to this type of ingot, without any special rolling strategy, then it can be seen from Fig.l that the width taper will increase as the thickness is reduced. The result is a plate with uniform thickness, but with a width that tapers down the length. 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, inversely proportionate to the ingot width taper, as illustrated by Fig. 2. This taper can then be rolled out by turning the ingot through 90° and rolling again in the width direction (broadsiding) in order to spread material and form a rectangular product, as shown in Fig. 3. 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.

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 and multiple roll gap changes in a rolling mill 24. The ingot or slab 20 is supported on a roller table 27 and moving in the direction of the arrow 30 enters a roll gap between work rolls 25 which roll gap has been pre-set. The slab 20 is rolled to a first thickness, as shown in Fig.4a to produce rolled slab 21, and the rolled slab 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, the screws are operated to reduce the gap between work rolls 25 for the next pass of rolled plate 21, typically by moving 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 reduced roll gap 34a in the direction of the arrow 32, but only a part 22 of the length of the rolled plate 21 is rolled again. When the required length of the rolled plate has been rolled to the new thickness, the rolling is stopped and the plate is reversed out of the mill. The plate is now formed of two parts 22, 21 of different thickness, as illustrated in Fig.4d. The screw is operated to change the roll gap by movement in the direction of the arrow 33 to the next required roll gap 34b and the process of rolling part of the length, stopping and reversing out of the roll gap is repeated. Each time, the work rolls are set to have a slightly smaller gap than the previous roll gap and the plate is rolled again.

For each iteration, part of a previously rolled section is not rolled again, but the subsequent roll pass finishes at a predefined boundary between the previously rolled thickness and the new thickness. Thus, the slab formed has a section of thickness of the first roll gap and a section of thickness 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 and 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. In each successive pass, the rolling is not over the full length, but stops at a boundary of formed between the immediately proceeding section and the most recent section. Rolling, reversing out and adjustment of the roll gap continue until a desired minimum thickness of the final section has been reached.

In each rolling pass, the rolled product becomes longer, 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 step along the taper is known from the different gaps produced by each different screw setting, allowing a calculation of how much longer the slab has become and so 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. This is performed by adjusting roll gap using the mechanical screw for multiple rolling stages of decreasing roll gap and increasing roll length until the entire length is rolled. The result is a plate with a thickness profile similar to a staircase. The resulting profile is illustrated in Figs.5a and 5b, which show the profile in plan view and cross section respectively. Outline 11 indicates the shape of the ingot, seen from above, before rolling and outline 12 the shape after rolling. Outline 13 shows the cross section before rolling and outline 14, the cross section after rolling.

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 rolling steel ingots, with both width and thickness tapers, into plate. The process may be used where the resulting plate has a thickness above 120mm, 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.