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Important aspects of Rolling of Hot Rolled Coil in Hot Strip Mill

Important aspects of Rolling of Hot Rolled Coil in Hot Strip Mill

Continuous cast slabs having thicknesses higher than 200 mm are used to make hot rolled strip. The transformation of the slabs into sheets / plates with a thickness of only some millimeters is the most important phase during flat steel rolling. The operations are carried out in a hot strip mill (HSM). Hot rolled coil is the lengthened steel sheet / plate (strip) which is produced in coil form for its easy handling and transport.

Hot strip mills in these days are either conventional hot strip mills or strip mills for rolling continuous cast thin slabs. The primary function of the conventional HSM is to reheat the semi-finished slabs to the rolling temperatures and then to roll them thinner and longer through a series of rolling mill stands driven by large motors and finally coiling up the lengthened steel sheet / plate (strip) for its easy handling and transport.

During the period of 1960s and 1970s, HSMs were designed as fully continuous mills or three quarter continuous mills featuring 5 to 7 roughing stands. Present day mills achieve annual production ranging 3 million tons (Mt) and 5.5 Mt in semi continuous set-up with 1 to 2 roughing stands.

Strips are produced in thicknesses upto 25 mm. Hot rolled coils are produced with an inside diameter of 750 mm on the coilers, with  an outside diameter of upto 2,600 mm and with the limitations of coil weight upto 22 kg per mm width. Hot rolled coils are used for cold rolling, and for strip slitting into smaller width coils and shearing into straight lengths sheets / plates. The range of HSMs for the rolling of hot rolled coils has undergone significant changes in the last few decades with minimum thickness of the hot rolled strips has been decreased from 1.8 mm to 2 mm to 0.8 mm to 1.2 mm. The strips of such thickness were earlier produced only in the cold rolling mills.

Several requirements are to be met by the HSM. These requirements are (i) high mill availability coupled with high productivity and high yields, (ii) low maintenance, (iii) low energy consumption, (iv) improved product quality by meeting close thickness and profile tolerances, and (v) flexible rolling schedules to ensure short delivery times and economical rolling of smaller lot sizes.

The basic equipments of a conventional hot strip mill are reheating furnace, roughing stand / stands, finishing stands, accelerated control cooling (ACC) of the strip, and coilers (Fig 1). After leaving the roughing stand / stands, the slab passes continuously through the finishing stands which progressively reduce the thickness. As the steel becomes thinner, it also becomes longer and moves through the rolls faster. Because different parts of the same piece of steel are traveling at different speeds through the different rolls, this process needs very close computer control of the speeds at each individual rolls of each stand. By the time it reaches the end of the mill, the steel can be traveling at speeds upto 20 m/sec. As the long strip of steel comes off the strip mill, it is coiled and allowed to cool.

Fig 1 Typical layout of a hot strip mill

The inter-stand facilities are also critically important for the production of hot rolled strip having good surface quality. Important inter-stand equipment includes (i) entry guides and exit guides, (ii) work roll cooling system, (iii) anti peeling device, (iv) roll gap lubrication system, and (v) inter-stand cooling and descaling systems. Close interplay of all these facilities is necessary to achieve an optimal result. Side guard featuring hydraulic width adjustment ensures exact positioning within minimum time. The strip guide areas are designed so that all wearing parts can be replaced quickly.

The inter-stand cooling efficiency is improved by optimized selection and arrangement of nozzles. The combination of roll gap cooling, roll gap lubrication and improved exit side cooling systems reduces the roll temperature. This results in a thinner oxide layer on the roll surface with less work roll peeling as a result. Lubrication inside the roll gap minimizes friction, hence enabling rolling force reduction by 20 % to 30 %. In this way, it is possible to redistribute the rolling force for optimizing the pass schedule and thinner final strip gauges. Added to this, chattering or vibrations in the stand are prevented which leads to longer roll service lives.

Hot rolled coils produce sheets or plates after the uncoiling of the coils, straightening, and shearing in a shearing line to straight length pieces of desired length. These sheets or plates are identified by their width being several times higher than of their thickness. They are having rectangular section in which length and width happens to be more than 100 times of the thickness. The sheets are defined as the rectangular sections with thickness upto 5 mm. Thickness of 5 mm and above lengths are called plates.

Hot rolled coils being rolled in HSM are of several steel grades.  In addition to the conventional steel grades such as low carbon steels, high carbon steels, and steels for cold rolling, hot rolled coils of special steels are being produced in the HSM. HSM is also to be capable of producing hot rolled coils of special steels such as line pipe steel, DP (dual phase) steel, TRIP (transformation induced plasticity) steel, HSLA (high strength low alloy) steel, IF (interstitial free) steel , and silicon (Si) steel etc.

A modern HSM normally has (i) high performance equipment, (ii) high quality of the strip surface with precise size, (iii) improved mechanical properties of the steel, (iv) high productivity of the mill and equipment, (v) high usable output, and (vi) low production costs. In the finishing mill of HSM, an important task is performed by a hydraulic arm, called the looper, placed in the middle between the two consecutive stands and whose purpose is to keep the strip tension at a constant value. This mechanical system is subject to particularly unstable dynamics which make the control issue tricky in the HSM.

One of the key objectives in the rolling of the strip in HSM is to achieve the target thickness with optimum quality in terms of crown and flatness of the strip leaving the mill. During thickness reduction, it is very important to have a constant reduction across the strip width in order to have a uniform elongation between the centre and the edge of the strip. If this condition is not met, an internal stress condition is generated, causing flatness defects (centre buckles or wavy edges).

With an increasing need for higher yield, process streamlining, and automation at the steel user end in the recent years, hot rolled coils are expected to meet these requirements. Because of this reason, quality requirements of hot rolled strip have turned more rigorous, with higher accuracy demanded not only on the properties of the steel, but also on such dimensional and shape requirements as thickness, width, flatness, and profile.

The shape of a rolled strip is characterized by its transverse section profile (crown) and by its flatness. The accuracy of the strip shape is an important parameter which determines the quality of the hot rolled strip. Strip shape is an indispensable factor in determining the market competitiveness of the rolled strip. Since the strip shape is a key indicator of quality, the shape control technology is the core technology of hot strip production. There are several factors which influences the shape of rolled strip in the HSM. Fig 2 shows a width wise distribution of strip, comprising strip crown indicating a centre height of strip, high spot resulting from local wear of roll, and edge drops. For meeting these requirements, sophisticated techniques are needed for rolling of hot rolled coils in the HSM.

Fig 2 Typical profile of a hot rolled strip

The first major operation in the HSM is the reheating of the slab. When the slab leaves the caster area, it is hot with a temperature above 900 deg C. The slab waits in the slab yard before being charged into the reheating furnace and its temperature is reduced, fluctuating between 100 deg C and 800 deg C. This slab is warm slab. The slab is called cold slab, when the slab is cooled to ambient temperature in the slab yard before being charged into the reheating furnace.

The technology as well as its operation is important for the reheating furnace. The operation of the reheating furnace is very important for the metal yield, environment emissions, and cost. Around 30 % of the variable cost of rolling a slab into a hot rolled coil is spent on the fuel gas. During the slab reheating in the reheating furnace, the requirement of fuel energy is substantially reduced by charging the reheating furnace with the slabs at high temperature (i.e. hot charging) and keeping the necessary reheating temperature as low as possible. The latter is done by reducing the heat loss in during the rolling in the HSM. Radiation losses are reduced using heat panels. When applying water spraying, as in the case of descaling, attention is also needed to be given for the convection losses.

The important issues concerning the reheating of the slab in the reheating furnace are that (i) there is direct flame contact in the reheating furnace which oxidizes the slab surface resulting into typically around 1 % of material loss due to the scale formation, and (ii) the inertia of a reheating furnace is high. Adjustment of the operation of the reheating furnace due to change of the slab temperature takes time. The discharge temperature can only be changed gradually.

The reheating furnace is not suitable for a precise, slab to slab adjustment of the discharging temperature in correspondence with the aimed entry and exit finishing mill temperature. This lack of flexibility is a drawback during the heating of the slab in the reheating furnace. Since it is not possible to change the discharge temperature from slab to slab, successive slabs are normally heated to a temperature sufficiently high to accommodate the slab which needs the highest discharge temperature. This is normally the slab which is going to be rolled to a small thickness (i.e. high heat loss) or with a high finish rolling temperature. As a result, the other slabs in sequence are heated to a higher temperature than what is needed. This not only causes too much energy input into the slabs, but it also affects the production rate since the slab is to wait on the roller table to cool down. Thus there is expenditure of the energy in the reheating furnace and there is also loss of the rolling capacity. The facilities provided in HSM to overcome this lack of flexibility in the reheating furnace are (i) transfer bar cooling, and (ii) induction heating.

Fundamental concepts applicable during the rolling of strip in HSM are (i) the arc of contact between the rolls and the material being rolled is a part of a circle, (ii) the coefficient of friction is constant in theory, but in reality it varies along the arc of the contact, (iii) the material being rolled is considered to deform plastically during the rolling, (iv) the volume of the material is constant before and after the rolling (in practice the volume can decrease a little bit due to close-up of pores), (v) the velocity of the rolls is assumed to be constant, (vi) the material only extends in the rolling direction and no extension in the width of the material, and (vii) the cross sectional area normal to the rolling direction is not distorted.

The main variables which influence the rolling process during rolling the strip in HSM are (i) roll diameter, (ii) deformation resistance of the material as influenced by metallurgy, temperature, and strain rate, (iii) material flow stress, (iv) friction between the rolls and the material being rolled, and (v) presence of the front tension and / or back tension in the plane of the strip.

Rolling of strip in HSM is a complex process where the quality of the product is influenced by a range of factors such as incoming material, mechanical and electrical equipment, operating parameters, lubrication, and automation and control strategies etc. The significant quality parameters are material thickness, material shape and surface, and the homogeneity of stress distribution. For optimized cost-efficiency and to maximize material usage, tight tolerances for the thickness are necessary, to enable the strip to be rolled down as closely as possible to the minimum permissible thickness. Product quality can only be effectively optimized if the mechanical, electrical and instrumentation equipment as well as the control strategy solution combine together well.

There are several factors which affect the scheduling of the production of coils. These are (i) product quality specifications,(ii)  process efficiency standards, (iii) productivity, and (iv) target delivery due date. Each slab has several important characteristics such as width, thickness, grade (chemical composition), charging temperature, drop out temperature, aggregate force (force needed to reduce the thickness of a slab), and gauge (required thickness of the strip which is to be produced), among others. The most important restrictions need smooth changes in four aspects namely (i) width, (ii) aggregate force, (iii) gauge, and (iv) residence time in the reheating furnace.

Rolling schedule plays a decisive role in rolling of strip in HSM. Traditionally, the most important factor is the evolution of the profiles of the rolls in the mills caused by their wear and thermal expansion. From this derives the `coffin’ schedule which means start narrow, quickly build up to wider material as the thermal crowns increase on the rolls, and gradually fade back to narrow as the rolls wear.

Rolling schedule has an important effect on the capability of mill. The rolling mill schedule includes reduction, speed, and temperature schedules. Reduction schedule determines rolling passes and the reduction per pass. Speed schedule determines the bite speed, through speed and maximum rolling speed without variation in acceleration and deceleration of the motor. Temperature schedule controls the temperature drop of mill house and finish rolling temperature by cooling water flow according start rolling temperature. The issues which are important for the rolling schedule are (i) the shape of the strip is good and the crown meets the specifications requirement, (ii) the yield of rolling mill, and (iii) the good performance of strip. The rolling schedule is to ensure that the strips produced in the HSM meets the requirements of dimensions, comprehensive properties and micro-structure of the strip.

The work rolls of the HSM are to withstand extreme service conditions. On leaving the reheating furnaces, the slab temperature is around 1250 deg C. During rolling, due to contact with the strip, the roll surface heats up from 50 deg C to 80 deg C (stationary conditions) to very high temperatures (500 deg C to 600 deg C in the initial stands) in only one second, water sprays subsequently cooling it back down to 80 deg C in around 4 seconds. These thermal changes promote severe thermal fatigue cycles which affect a depth of 1 mm to 2 mm. Longitudinal and circumferential compressive stresses are generated as the roll surface heats up until the roll surface yields plastically (the high temperature yield strength and the thermal expansion coefficient of the alloy determining this point). In the cooling half of the cycle, tensile stresses are generated and further yielding occurs. Fig 3 shows the thermal stresses on the work roll surface.

Fig 3 Thermal stresses on the work roll and their effects

The thermal stresses on the work rolls are so high that a pattern of cracks develops after a short working period. These cracks are mostly deep in the rolls at the initial stands, but mostly shallow at the last finishing stands (Fig 3). These fire cracks also grow and branch due to the high stresses generated in each revolution in the contact between the work roll and the back-up roll (Hertzian stresses higher even than 2,000 MPa). The Hertzian stresses increase with the rolling load as the strip cools in contact with the rolls of the different stands of the mill.

Further, the work roll surface also suffers oxidation and abrasive wear processes. Wear is produced by the compressive rolling load in combination with the slip between roll and strip along the contact angle and especially by the presence of an oxide scale on the surface of the strip. This scale is composed of three layers with the outside layer is Fe2O3, the middle layer is Fe3O4 and the inner layer is FeO, their average hardnesses at room temperature being 1,000 HV, 450 HV and 350 HV respectively. Furthermore, the thickness of these layers is dependent on the strip temperature. The strip under the roughing stands with temperatures in the range of 1,150 deg C to 1,250 deg C has a substantial content of abrasive Fe2O3 scale, while under the last finishing stands with temperatures in the range of 850 deg C to 950 deg C soft FeO scale predominates. This is the main reason underlying the different wear patterns normally observed at the different stands of the HSM.

Also, high-speed impacts with the cold leading and trailing edges of the strip occur every 2 minutes to 3 minutes. Furthermore, the work rolls can also suffer bending and torsion stresses which attain maximum values near the rolling necks.

The work roll performance in HSM depends on the tons rolled per millimeter at the time of roll discard after reaching of the minimum roll diameter. It also depends by the amount of wear or dressing needed after each roll run. At the end of the run, the surface of the work roll is non-destructively inspected and is then ground to eliminate all the defects (surface cracks) and the outermost deteriorated layer. After this, the roll is inspected again before being mounted in the mill. Each work roll pair is used until a minimum diameter is achieved. Hence, any improvement of the rolling roll behaviour in the mill has a direct impact on major cost aspects. This is because of (i) better strip quality (surface and shape), (ii) higher productivity (tons rolled before discard), (iii) reduced number of rolling roll changes and lower roll inventory, and (iv) improved working conditions of HSM rolling rolls.

There are several methods of improving the strip mechanical properties. These methods include alloying, heat treatment, controlled rolling and accelerated control cooling. The most promising for the improvement of the strip quality is controlled rolling with the subsequent accelerated control cooling (ACC). During the ACC, cooling is carried out from the rolling heat by removing the heat by the cooling environment (water) supplied to the hot surface of strip. In this case, the method and the feed rate of water on the strip surface considerably affect the final properties of the strip.

The ACC system is designed as a laminar flow system on both top and bottom, incorporating a line side head tank system. The cooling zone is divided into the required cooling banks  which consists of intensive cooling banks for faster cooling, normal cooling banks for regular cooling, and the cooling banks for fine temperature control in order to achieve the desired cooling patterns and coiling temperatures for the DP and the TRIP steels.  The combination of laminar cooling and edge masking system prevents excessive cooling of the strip edges, thus minimizing stress differences across the strip width.

Several steps involved during the processing of slab to hot rolled strip in the HSM are complex and involve mechanical and automation technologies. Hot rolling of the strip in a HSM needs not only mechanical solutions but also appropriate control technologies. The process of rolling in the HSM can be controlled though a standard software and automation architecture which includes four automation levels namely levels 0, 1, 2, and 3 respectively.

The use of advanced control and modeling solution for HSM has been subject to several developments in various directions in the past 40 years to 50 years. These include (i) the use of multivariable control techniques which has been proposed for the finishing mill since the 1970s and now it is considered a consolidated tool for controlling the generic rolling stand together with the downstream looper or the downstream coiler, (ii) development of various models in order to predict the material characteristics according to the material temperature and the rolling process and application of controls for regulating the coiling temperature, (iii) development of advanced control techniques which are applied in order to compensate friction phenomena, (iv) development of models and controllers in order to improve the material flatness and profile, and (v) development of steering control techniques which have been recently introduced in order to increase the productivity levels by reducing the probability of cobbles.

A reliable automation system is necessary for the high performance of the mechanical and the hydraulic equipment of the mill. The automation system is to decide the mill setup which calculates the schedule of rolling of the high quality strip taking into account mill constraints, energy consumption, deterioration of the equipment and the mill productivity. The schedule of rolling is calculated by the mathematical models for each slab to be rolled in order to take into account the variance inherent to the mill, for example, possible differences in the temperature between the two constitutive slabs entering the mill and the impact of this difference has on the rolling force needed to achieve the same final thickness, or the wear of the work roll is subject to, slab by slab, and its impact of the thickness of the strip being rolled.

The automation system is not the sole determinant of performance of the HSM. However, for any given configuration of mechanical and electrical equipment, the potential performance of the mill is only being achieved with high-performance control and automation. The basic data for the automation system include (i) geometric and physical data of the entry slab (dimensions and steel quality), (ii) target data for the strip (thickness, width, temperature, and profile etc.) and (iii) mill data and the limits of the HSM.

The automation technology applied in the HSM is normally divided in four levels referred to as levels 0, 1, 2, and 3, respectively (Fig 4). All these automation levels need to cooperate hierarchically in order to achieve the best performances. For the highest productivity levels of the mill, a number of control technologies, mathematical models of physical phenomena, and optimization algorithms are needed to be implemented in the mill.

Fig 4 Level of automation in hot strip mill

References to level 1 automation system and to the actuators are calculated by the mathematical models for every strip to be rolled in order to take into account the variance inherent to the mill. For example, there can be possible difference in temperature between the two slabs entering the mill, and the impact of this difference on the rolling force needed to achieve the same final strip thickness, or the wear of a work roll is subject to, strip by strip, and its impact on the strip thickness.

Level-2 automation takes particular care to track the strip from entry of the slab to the mill to the time hot rolled coil is produced and leave the HSM. Tracking of the every strip being rolled takes into account all the strips throughout the entire mill. It enables measurement acquisition and manages all the operations in which mill set up and adaption is to run. There are three factors which contribute to mill set up. These are (i) rolling strategies, (ii) mathematical models and (iii) model adoption.

The level 3 functions mainly as a scheduler of the hot strip mill. It takes the order book for the mill and organizes it into rounds of, typically, 100 work pieces to 200 work pieces which comply with the scheduling rules developed for the mill. As well as the HSM itself, level 3 takes account of upstream and downstream processes and stock areas. The scheduling rules used in HSM are, basically, a global model of the rolling mill which enables the scheduler to organize the round so that the required quality parameters can be achieved within the constraints imposed by the mill equipment.

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