Primer on Flat Rolling

By John G. Lenard

Primer on Flat Rolling is a fully revised second edition, and the outcome of over three decades of involvement with the rolling process. It is based on the author's yearly set of lectures, delivered to engineers and technologists working in the rolling metal industry. The essential and basic ideas involved in designing and analysis of the rolling process are presented.

The book discusses and illustrates in detail the three components of flat rolling: the mill, the rolled metal, and their interface. New processes are also covered; flexible rolling and accumulative roll-bonding. The last chapter contains problems, with solutions that illustrate the complexities of flat rolling.

New chapters include a study of hot rolling of aluminum, contributed by Prof. M. Wells; advanced applications of the finite element method, by Dr. Yuli Liu and by Dr. G. Krallics; roll design by Dr. J. B. Tiley and the history of the development of hot rolling mills, written by Mr. D. R. Adair and E. B. Intong.

Engineers, technologists and students can all use this book to aid their planning and analysis of flat rolling processes.

• Provides clear descriptions for engineers and technologists working in steel mills
• Evaluates the predictive capabilities of mathematical models
• Assignments and their solutions are included within the text

• Language: English
• Publisher: Elsevier Science
• Release date: Dec 4, 2013
• ISBN: 9780080994123

Book preview

1

Introduction

In this chapter, the topic of flat rolling of metals is introduced. The products of flat rolling, strips and plates, are defined in terms of their geometry; the ratio of their thickness to width is much less than unity. Strips are substantially thinner than plates. Rolling of strips and plates is generally referred to as flat rolling. The objectives of the process – that of reducing the thickness of the work piece, increasing its length and thereby changing its mechanical and metallurgical attributes – are stated in this introductory chapter. The temperatures at which the hot, warm and cold rolling processes are performed are defined. Each of the processes is presented and discussed in general terms. The equipment, the hot strip mill, mini-mills, Sendzimir mills, planetary mills and the cold rolling mill are shown and described, along with several mill configurations. The continuous casting process, as applied in the hot rolling industry, is also shown. New, recently developed equipment is described, as well. A selected list of books and publications, dealing with the theory of plasticity, plastic forming of metals and specifically the flat rolling process, is followed by some general conclusions.

Keywords

flat rolling; hot; warm and cold rolling; strips; plates; rolling mill types

1.1 The Flat Rolling Process

The mechanical objective of the flat rolling process is simple. It is to reduce the thickness of the work piece from the initial thickness to a pre-determined final thickness. This is accomplished on a rolling mill, in which two work rolls, rotating in opposite directions, draw the strip or plate to be rolled into the roll gap and force it through to the exit, causing the required reduction of the thickness. As these events progress, the material’s mechanical attributes change. These in turn cause changes to the metallurgical attributes of the metal, which, arguably are of more importance as far as the product is concerned. A schematic, three-dimensional diagram of the back-up rolls and the work rolls is shown in Figure 1.1 where a single-stand, four-high mill is depicted; this may be a single-stand roughing mill.

Figure 1.1 A schematic diagram of a single-stand, four-high set of rolls.

Figure 1.1 shows the back-up rolls, the much smaller work rolls, the strip being rolled and the roll separating forces acting on the journals of the back-up roll bearings, keeping the centre-to-centre distance of the bearings as constant as possible¹. As will be demonstrated in Chapter 10, the energy requirements of the process may be decreased when small diameter work rolls are used. The drawback of that step is the reduced strength of the work roll which necessitates the use of the massive back-up rolls to minimize the deflections of the work roll.

1.1.1 Hot, Cold and Warm Rolling

While the rolling process may be performed at temperatures above half of the melting point of the metal, termed hot rolling, or below that temperature, in which case one deals with cold rolling, the division into these two categories should not be considered as being cast in stone. There is a temperature range, beginning below and ending above the dividing line between hot and cold rolling, within which the process is termed warm rolling and in some specific instances and for some materials this is the preferred process to follow. These processes lead to mechanical and metallurgical changes of the attributes of the work piece, which are not possible to achieve in either the cold or the hot flat rolling regimes.

1.2 The Hot Rolling Process

Hot rolling of metals is usually carried out in an integrated steel mill, on a Hot Strip Mill, or since some changes were introduced in the last couple of decades, on mini-mills². Both have advantages and disadvantages, of course, such as capital costs, flexibility, quality of the product and danger to the environment.

A schematic diagram of a traditional hot strip mill is depicted in Figure 1.2, showing the major components.

Figure 1.2 The schematic diagram of a traditional hot strip mill.

There are several basic components in the traditional hot strip rolling mill. In what follows, these are discussed briefly³.

1.2.1 Reheating Furnace

The reheating furnace constitutes the first stop of the slab after its delivery from the slab yard. The slab is heated up to 1200–1250°C in the furnace to remove the cast dendrite structures and dissolve most of the alloying elements. The decisions to be made in running the reheat furnace in an optimal fashion concern the temperature and the environment within. If the temperature is higher than necessary, more chemical components will enter into solid solution but the costs associated with the operation become very high and the thickness of the layer of the primary scale will grow. If the temperature is too low, not all alloying elements will enter into solid solution, affecting the metallurgical development of the product, and the likelihood of hard precipitates remaining in the metal increases. As well, thinner layers of scale will form, a fairly significant advantage. A judicial compromise is necessary here and is usually based on financial consideration. The cost savings associated with a one-degree reduction of the temperature within the furnace can be calculated without too many difficulties; the changes to the formation of solid solutions may be estimated but the annual savings may well be significant.

Primary scales of several millimetres thickness form on the slab’s surface in the reheat furnace. The thickness of the scale may be reduced by providing a protective environment within the furnace, albeit at some increased cost. As the furnace doors open and the hot slab slides down on the skids to the conveyor table, the instant chilling, caused by the water-cooled skids, causes marks that are often noticeable on the finished product. As well, fast cooling of the surfaces and especially of the edges is also immediately noticeable, indicating a non-uniform distribution of the temperature within the slab and leading to possibly non-homogeneous dimensional, mechanical and metallurgical attributes.

1.2.2 Rough Rolling

Before the rolling process begins, the scale is removed by a high-pressure water spray and/or scale breakers. The slab is then rolled in the roughing stands in which the thickness of the slab is reduced from approximately 200–300 mm to about 50 mm in several passes, typically four or five. The speeds in the rougher vary from about 1 m/s to about 5 m/s. In the roughing process the width increases in each pass and is controlled by vertical edge rollers. The vertical edgers compress and deform the slab somewhat, causing some thickening which is corrected in the subsequent passes. A large variety of roughing mill configurations is possible, from single-stand reversing mills to multi-stand, one-directional mills, referred to as roughing trains. These usually have a scale breaker as the first stand where the mill deforms the slab sufficiently just to loosen the scale, which is then removed by the high-pressure water jets. Roughing scale breakers are usually vertical edgers, capable of reducing the width of the slab by up to 5–10 cm and causing stresses at the steel surface-scale layer interface which then separate the scales. Roll diameters are near 1000 mm. The rolls are usually made of cast steel or tool steel⁴. Roughing stands are either of the two- or four-high configurations. At the end of the rough rolling process, the strip is sent to the finishing mill along the transfer table where it is referred to as the transfer bar. The temperature of the slab in the roughing stands is high enough so that the transfer bar is fully recrystallized, containing strain free, equiaxed grains. In general, though, the grain structure at the end of the rough rolling process seems to have little influence on the structure by the time the strip has passed through several stands of the finishing mill.

1.2.3 Coil Box

Not shown in Figure 1.2 is a device – an invention by the Steel Company of Canada and first installed in the early 1970s in Stelco’s Hilton Works – called the coil box⁵, placed between the roughing mill and the finishing train, in place of the transfer table. Since that time, several integrated steel companies have installed the coil box in their hot strip mills. A photograph of the coil box is shown in Figure 1.3.

Figure 1.3 The coil box. Source: Courtesy The Steel Company of Canada.

When the words Coil Box are entered into Google⁶, a plethora of information is found, including the possibility of watching a video of the coil box in motion. A detailed description of the events when the steel arrives to the coil box and when it is within the coil box are also available on-line.

The transfer bars, exiting from the roughing stand are formed into coils at the coil box, a patented design of the Steel Company of Canada. The coil box consists of two entry rolls, three bending rolls, a forming roll, two sets of cradle rolls, coil stabilizers, peeler, transfer arm and pinch rolls. The adoption of a coil box configuration has several advantages:

• it reduces the overall length of the mill line;

• it increases the productivity;

• it enlarges the strip width and the length to be rolled and

• it eliminates the thermal rundown along the strip length when compared to the conventional HSM.

Thus, uniform temperature and constant rolling speed conditions are maintained. On uncoiling from the coil box, the transfer bars are end-cut, processed through high-pressure descaling sprays, and then they are ready to enter the finishing stands. With the introduction of advanced high-strength steels such as Hot Roll Dual Phase steels⁷, the benefits of the coil box are even more significant in providing uniform mechanical properties throughout the length of the coil⁸.

1.2.4 Finish Rolling

When the transfer bar, now coiled up in the coil box, reaches the appropriate temperature, it is uncoiled and is ready to enter the last several stands of the strip mill, the finishing train. The crop shear prepares the leading edge for entry and the transfer bar enters the first stand, assisted by edge rollers. Its velocity is in the range of 2.5–5 m/s⁹. The finishing train in the strip mill is traditionally composed of five to seven tandem stands. The roll configuration is usually four-high, employing large diameter back-up rolls and smaller diameter work rolls. The entry of the strip into the first stand is carefully controlled and is initiated when the temperature is deemed appropriate, according to the draft schedule, which is prepared using sophisticated off-line mathematical models. These determine the reductions and the speeds at each mill stand as well as predicting the resulting mechanical and metallurgical attributes of the finished product. After entry into the first stand, the strip is continuously rolled in the finishing mill. At the entry to the finishing mill, the temperature of the strip is measured and at the exit, both temperature and thickness are measured; the thickness at the exit from each intermediate stand is estimated using mass conservation¹⁰. In some modern mills there are several optical pyrometers placed along the finishing train. The Automatic Gauge Control (AGC) system uses the feedback signals from several transducers to control the exit thickness of the strip. The finishing temperature may also be controlled by changing the rolling speed. However, only small variations of the rolling speed are possible without causing tearing, if the speed of the subsequent mill stand is too high, or buckling, referred to as cobble of the strip¹¹, when the speed there is too low. On some newer and more modern strip mills, interstand cooling and/or heating devices have been installed, which minimize the temperature variation across the rolled strip and thereby increase the homogeneity and the quality of the product. As the thickness is reduced the speed must increase, as demanded by mass conservation, and the speeds in the last stand may be as high as 10–20 m/s. The rolls of the finishing mill are cooled by water jets strategically placed around the rolls. Without cooling, the surface temperature of the work rolls would rise to unacceptable levels. It has been estimated that when in contact with the hot strip, the roll surface temperatures could rise to as high as 500°C at a very fast rate. Of course, the roll surface would cool during its journey as it is turning around and is subjected to water cooling, but the thermal fatigue it experiences accelerates roll wear and is, in fact, one of the major contributors to it. It is possible to measure roll surface temperatures by thermocouples embedded in the roll, with their tips positioned close to the surface¹². A mathematical model would then be necessary to extrapolate the temperatures to the surface. There are usually scale breakers before the first stand of the finishing train, consisting of one or two sets of pinch rolls, exerting only enough pressure on the strip to break off the scale. The strip exits from the finishing train at a thickness of 1–4 mm. The Hylsa steel mill in Monterrey, Mexico produces hot rolled strip of 0.91 mm thickness. Bobig and Stella (2004) describe the semi-endless rolling and ferritic rolling processes. These, introduced in the thin slab rolling plant EZZ Flat Steel in Egypt, produce 0.8 mm-thick coils. The ferritic rolling leads to reduced scale growth and lower roll wear.

During the last decade the materials used for the rolls on the hot strip mill were changed from chill cast to tool steels, reducing roll wear in a most significant manner¹³. There have also been reports of significant changes of the coefficient of friction in the roll gap after the switch of roll materials. Tool steels rolls, once implemented correctly, do provide benefits that offset their higher costs. The impact of lubricant interactions with these new roll chemistries has not been fully explored (Nelson, 2006).

1.2.5 Cooling

After exiting the finishing mill, the strip, at a temperature of 800–900°C, is cooled further under controlled conditions by a water curtain on the run-out table. The run-out table may be as long as 150–200 m. Cooling water is sprayed on the top of the steel at a flow rate of 20,000–50,000 gpm; and on the bottom surface at 5000–20,000 gpm (1 gpm=4.55 l/min). The purpose of cooling is, of course, to reduce the temperature for coiling and transportation, but also to allow faster cooling of the finished product, resulting in higher strength. The cooling process plays a major role in the thermal–mechanical schedule, designed to affect the microstructure of the product.

1.2.6 Coiling

At the exit of the run-out table, the temperature of the strip is measured and the strip is coiled by the coiler. After further cooling, the steel coils are ready for shipping.

1.2.7 The Hot Strip Mill

A photograph of a hot strip mill of Dofasco Inc. is shown in Figure 1.4. A pair of work rolls is visible, stored in the foreground of the figure and ready to be placed in the stands¹⁴.

Figure 1.4 The seven-stand finishing mill of Dofasco Inc. Source: Courtesy Dofasco Inc.

1.3 Continuous Casting

Irwing (1993) describes the history of the development of continuous casting and identifies Mannesmann AG, where a production plant went into operation in 1950. A continuous casting plant was installed at Barrow Steel in Great Britain in 1951. The essential idea of the process is simple: molten steel is poured into a water-cooled, oscillating mould. The cooled copper wall of the mould solidifies the outer layer of the steel and as the steel is moving vertically downward, the solidified skin thickens. As the steel leaves the mould, it is cooled further by water sprays. The solidifying steel is supported by rollers, which prevent outward bulging.

The continuous casting process replaced the ingot casting several decades ago and succeeded in increasing productivity. The complete continuous casting process is shown in Figure 1.5. The figure shows the ladle into which the molten steel is poured. From the ladle the steel is metered into the tundish and from there it enters the water-cooled, oscillating mould. As the steel strand exits the mould, it is solidifying further; an indication of the solidification front is also shown in Figure 1.5. Using the withdrawal rolls and the bending rolls, the now solid but still very hot strand is straightened and cut to pre-determined sizes by the cut-off torch.

Figure 1.5 Continuous slab casting. Source: Groover (2002); reproduced with permission.

There are two possible subsequent activities at this point. The slabs may be allowed to cool and are then stored in the slab yard, retrieved as needed by customers and reheated in the reheat furnaces and rolled, in the hot strip mill, as depicted in Figure 1.3. Alternatively, they may be rolled directly, as shown in Figure 1.6.

Figure 1.6 Continuous casting and direct rolling. Source: Following Pleschiutschnigg et al. (2004).

1.4 Mini-Mills (See Also Chapter 2)

The American Iron and Steel Institute’s website gives the following definition for mini-mills:

Normally defined as steel mills that melt scrap metal to produce commodity products. Although the mini-mills are subject to the same steel processing requirements after the caster as the integrated steel companies, they differ greatly in regard to their minimum efficient size, labour relations, product markets, and management style.

Currently in the United States 52% of the steel is rolled by 20 integrated steel mills and 48% by more than 100 mini-mills. The integrated mills roll approximately 400 tons/h while the mini-mills are capable of 100 tons/h.

Information is also available from Wikipedia, a web-based encyclopaedia. It identifies mini-mills as secondary steel producers. Also, it mentions NUCOR as one of the world’s largest steel producers, which uses mini-mills exclusively.

A very impressive number (79%) of mini-mill customers expressed satisfaction with their suppliers¹⁵.

The website www.environmentaldefense.org gives information regarding the recycling activities of mini-mills, stating that they conserve 1.25 tons of iron ore, 0.5 tons of coal and 40 lbs of limestone for every ton of steel recycled.

1.5 The Cold Rolling Process

The layers of scales are removed from the surfaces of the strips by pickling, usually in hydrochloric acid. This is followed by further reduction of the thickness, produced by cold rolling. Essentially there are three major objectives in this step: to reduce the thickness further, to increase the rolled metals’ strength by strain hardening and to improve the dimensional consistency of the product. An additional objective may be to remove the yield point extension by temper rolling, in which a small reduction, typically 0.5–5%, only is