Thin Slab Casting and Rolling
Thin Slab Casting and Rolling
For the production of flat products, liquid steel is generally cast in form of slabs usually in the thickness range of 150 mm to 350 mm in the continuous slab casting machines. These slabs are inspected, scarfed and then reheated in slab reheating furnace to the rolling temperatures before being rolled to hot rolled coils in a semi continuous or continuous hot strip mills. Development of thin slab casting and rolling (TSCR) technology is a step forward to reduce the number of process steps in the production of hot rolled coils (HRC). Originally TSCR technology was developed with the primary goal of reducing the investment and production costs but today it has become one of the most promising production routes to maintain steel as a leading material in technological application and it is being considered as the technology which has reached a high degree of maturity. Casting speed of 6.0 meters per minute (m/min) for slab thickness of 50/55 mm is quite common these days.
Initially, only commercial quality plain carbon steels were being cast through thin slab casting route. But presently most of the steel grades including low, medium and high carbon steels, HSLA (high strength low alloy) steel grades, line pipe steel grades, and steel grades for automotive application including IF (interstitial free) grades can be cast through thin slab casting route. In fact this technology has brought paradigm shift in steel technology of casting and rolling. The thin slab casting and rolling technology was made possible because of the several improvements in casting and rolling processes which include (i) design of the mould, (ii) hydraulic mould oscillations, (iii) use of electromagnetic brakes (EMBR), (iv) use of high pressure descaler and roller side guide (edger) in the mill, (v) dynamic liquid core reduction (LCR), (vi) mould powder quality, and redesigned SEN, and (vii) water spray cooling.
The major advantages of TSCR technology over thick slab casting and hot rolling include (i) reduction in capital cost, (ii) reduction in manpower, (iii) reduction in required floor space, (iv) improvement in the yield of finish product from liquid steel, (v) reduction in the specific fuel consumption, and (vi) reduction in the specific power consumption.
The implementation of TSCR concept did not achieve any success till mid eighties due to the numerous technological challenges associated with the technology. The first breakthrough in this direction was achieved in October 1985 by SMS Schloemag-Siemag when the first thin slab of 50 mm thickness was cast in a pilot plant in Kreuztal-Buschhutten. This success was achieved with a patented funnel shaped mould and an optimized submerged entry nozzle. SMS Schloemag-Siemag gave this technology the name of ‘Compact Strip Production (CSP)’. The first CSP plant was ordered in late 1987 and was commissioned at Nucor Steel, Crawfordsville, Indiana in July 1989. Within few days of commissioning there was a major break out in the casting machine because of inexperienced operators. This has an effect on the speedy acceptance of this technology. Almost at the same time, the In-line Strip Production (ISP) process was under intensive development by Mannesmann Demag and Arvedi group. Development of ISP started in 1988 based on an Arvedi-Mannesmann patent. In 1992, the prototype ISP plant was built by Arvedi at its Cremona works, where most of the development of this process has been carried out. In 1994-95 the prototype plant completed the first phase of its development. Later based on the ISP technology, Acciaieria Arvedi has developed a new thin slab casting/endless rolling process under the brand name Arvedi ESP. With this process 0.8 mm thick strip is being produced. Voest Alpine (VAI) of Austria (now Primetals technologies) and Danieli of Italy have also developed thin slab casting and rolling technologies. The technology of VAI is known as Continuous Thin Slab Casting and Rolling Technology (CONROLL) and produces slabs up to thickness of 125 mm. It is more of medium thickness slabs. The technology developed by Danieli was known as Continuous flexible Thin Slab Rolling (fTSR) and produces slabs in the thickness range of 70 mm to 90 mm. Implementation TSCR technologies got a major boost after China and India started expanding their steel production in a massive way. Today a three strand TSCR plant based on CSP technology has been installed at ESSAR, Gujarat, India (now Arcelormittal Nippon Steel India Limited).
TSCR process has undergone an extremely dynamic development mainly with regard to output per plant, availability, conversion costs, range of steel grades produced, near-net-shape dimensions, strip geometry, quality parameters, and surface characteristics. Today, TSCR technology is used to produce not only steels in the lower carbon and medium carbon range, but also stainless ferritic and austenitic grades and grain oriented silicon steels. The upstream stages and the downstream hot strip processing facilities have also seen further development, with plant engineering and technology adapting to the products of the CSP plant, for example, hot strip thicknesses down to around 1 mm.
Metallurgical and other important features of TSCR process
While processing the steel in TSCR process from tundish to coiler there are several metallurgical and other important features of the process as described below.
- Rapid solidification of the thin slab refines the dendritic structure. Correction in dendritic structure helps to more homogenous structure.
- Non-metallic inclusions are small and globular, retain their shape during hot-rolling and contribute to isotropic properties (such as toughness, and bendability etc.).
- All added micro-alloying elements remain in solution. Because of the high temperature of the cast slab prior to hot rolling, premature precipitation is avoided.
- For minimizing the difficulties of casting in the peritectic region, the carbon content of many micro-alloyed steels is restricted to between 0.05 % and 0.06%. This restriction benefits toughness and weldability.
- The high temperature of the slab during bending and unbending minimizes the tendency to form transverse cracks.
- The temperature in the equilibrating furnace depends on the micro-alloying element and is designed to keep the micro-alloy in solution.
- Direct charging is the main factor which reduces the energy consumption during hot rolling.
- In rolling thin slabs, the deformation in the initial passes often exceeds 50 %. Heavy deformation at high temperatures is essential to refine coarse austenitic grains (over 1000 micrometers) by re -crystallization.
- The refinement and uniformity of austenitic grains is a prerequisite for a fine ferritic structure down to 4 micrometers to 5 micrometers.
- Accelerated cooling on the run-out table and under cooling of the austenite further refines the ferrite.
The production process and typical level of temperature evolution in the TSCR process is shown in Fig 1.
Fig 1 Typical level of temperature evolution during TSCR process
Faster solidification in the TSCR provides more uniform finer structures. Fine secondary dendrite arm spacing (SDAS), achieved due to rapid solidification in casting, and promotes a more homogenous structure with less micro-separation structure than the conventional casting. Micro-segregation is the segregation between the dendritic arms where minimum solute concentration is at the centre of the dendrite arms and the maximum solute concentration is between the arms. In fact, SDAS is a measure of the non-homogeneity in slab continuous casting. In thin slab casting, ‘liquid core reduction’ (LCR) system is used. Liquid core reduction allows for the thickness of strand below the mould to be reduced while the core is still in liquid phase. Generally the first segment in the strand guiding system can be adjusted to obtain the desire reduction in thickness of strand. The different TSCR processes are described below.
Compact Strip Production
The basic concept in the CSP technology is the achievement of maximum cost-efficiency through the linking of the three process stages namely (i) casting, (ii) temperature equalization, and (iii) hot rolling. The liquid steel after steel making is teemed into the tundish of the continuous casting machine (CCM). In this technology, the liquid steel is cast into slab of the desired thickness ranging from 50 mm to 90 mm. The slab is then sheared to the proper length and transported to the tunnel or equilibrating furnace normally set at a temperature of 1150 deg C. At this point, the slab shows an austenite grain size of 500 micrometers to 1000 micrometers. After the 20 minute residency time in the furnace the slab leaves the furnace and is crop sheared. The hot thin slab then enters the finishing mill at around 1000 deg C. The slab is rolled into hot strip (thickness ranging from 0.8 mm to around 16 mm and width ranging from 800 mm to 1880 mm) as it passes through the finishing mill of 5, 6 or even 7 stands. The hot strip after rolling enters the run out table (ROT) where it undergoes laminar cooling to the coiling temperature. It is then coiled to room temperature. The scheme of the CSP process is shown in Fig 2.
Fig. 2 Scheme of Compact Steel Production process
The main elements of CSP process are ladle turret, mould, strand guide system, pinch roll unit, pendulum shear, heating furnace with transfer car to connect two casting stands, rolling mill with 5 to 7 stands, strip cooling and coilers. The process has flexible setting of the slab thickness during casting through liquid core reduction, enabling adaptation to final strip thickness and of the casting thickness and casting speed. Hot strips of 1 mm thickness can be comfortably hot rolled by this technology. CSP uses profile and flatness control systems adopting the well known CVC technology for adjustment. The capacities of one strand plant are upto 1.5 million tons per annum (Mpta), two strands plant is upto 3 Mpta, and 3 strands plant upto 4 Mpta. The schematic layout of the three strand CSP plant is shown in Fig 3 and different concepts of the CSP plants are shown in Fig 4..
Fig 3 Schematic layout of the three strand CSP plant
Fig 4 Different concepts of the CSP plants
The heart of the CSP process is the patented funnel shaped 1100 mm long chromium-zirconium-copper mould (Fig 5). The feeding of the liquid steel is done through a submerged entry nozzle (SEN) into the confines of a narrow mould cavity by flaring the inlet of the mould in the form of a funnel. This funnel terminates approximately in the middle of the mould length beyond which, the broad side walls of the moulds are parallel and are separated by a thickness which is equal to the thickness of the thin slab. The sides of the mould wall are tapered to compensate for the shrinkage of the solidifying steel. On account of the rapid solidification of the thin slab, it is necessary to provide strand guidance over a length of around 4.5 m to 5 m only even at a high casting speed of 6 m/min.
Compared with its original design, the CSP casting machine has seen remarkable developments with regards to the configuration of the funnel-shaped mould, the length of the strand guide, the technological control loops, as well as its flexibility in terms of throughput and casting thickness. In the casting plants which are in operation today, the strand guide length has been increased from around 6.0 m to 9.7 m (Fig 5). This development was implemented while maintaining the vertical concept with all its equipment-related and metallurgical advantages. A decisive step was the further development of the mould from the U-frame design to the double-frame or O-frame mould. This innovative mould is secured at its centre, while at the same time, markedly increasing the stability of the entire structure. The frame has lateral windows to accommodate the yoke if and when an electromagnetic brake (EMBR) is used.
Fig 5 Schematic diagram of a CSP thin slab casting machine and its progressive development
The technological control loops of the CSP caster were systematically extended with a view to attaining maximum operational reliability and optimum product quality. This mainly relates to mould level control, hydraulic mould oscillator, liquid core reduction (LCR), and measurement of the heat flux density in the mould and breakout early detection, as well as the dynamic control of final solidification for the purpose of ensuring a high slab entry temperature into the tunnel furnace. The direct linking of casting and rolling creates additional potential with regard to throughput capacity and final dimensions. LCR enables the optimal slab thickness to be set in order to achieve limit dimensions, the maximum throughput capacity, or the required degree of shaping while casting is underway.
The development of CSP rolling mill has progressed to meets the demands of the market. The best solution for meeting various requirements and including a wide range of finished products is the compact rolling mill layout featuring six or seven stands depending on product mix. In order to obtain minimum finished strip thicknesses down to 0.8 mm, different slab entry thicknesses are used and the work roll diameters and roll materials optimized such that two or three different roll diameters are used. The CVC (continuously variable crown) technology, which has already proved successful in conventional hot-rolling mills, has been further enhanced with the development of CSP finishing mills. The result is the so called ‘CVC Plus’ process, which features a markedly wider adjustment range for profile control of upto 190 % of the CVC setting range.
The use of more efficient inter-stand cooling systems enables the ferritic rolling of ultra low carbon steels as well as the thermo-mechanical rolling of HSLA steels. A flying shear installed ahead of a rotor coiler and/or of two down-coilers offers the possibility of semi-endless rolling of thin strip. Rapid cooling systems downstream of the last stand or in the rear part of the cooling line, offer greater flexibility in implementing temperature- time curves to achieve specific strip properties and to process multi-phase steels.
The ability to roll down to final thicknesses of less than 1.5 mm is a particular feature of the CSP process, and technology packages have been specifically developed for this, however, depending on ingoing thicknesses, temperature evolution, distribution of reduction and rolling speed, minimum final thicknesses down to 0.8 mm can be achieved at austenitic finish rolling temperatures.
Compared with the conventional hot-strip production, the achievable thicknesses are upto 50 % thinner depending on the steel grade involved. This, in turn, increases the requirements in the CSP rolling mill, especially in terms of force and energy demand, roll wear predominantly in the late stands, and strip flatness. Roll gap lubrication is adopted for minimizing wear, reduction of roll-separating force and torque, as well as for improvement of the strip surface. For flatness measurement and control, segmented loopers are installed in several mills which detect, not only the inter-stand tension, but also the strip tension distribution over the width, and that activate the work roll bending function for the purpose of flatness control.
Despite these measures, however, for very thin strip, it is difficult to achieve high strip flatness once the strip has cooled down. Strip, though still flat on the run out roller table because it is under tension, can show edge waviness during later uncoiling. Edge masking, was developed to safeguard the flatness of the cooled strip. Adjustable guide plates under the laminar flow cooling groups keep water away from the strip edges and enable the flatness to be improved from 250 I-units to 25 I-units (I-Units is an exacting quantitative flatness measurement. It is a dimensionless number which incorporates both the height and peak to peak length of a repeating wave).
The control loops installed throughout a CSP rolling mill are constantly updated to the actual technological status. They serve not only to determine the rolling strategy, to calculate the pass schedule and to preset the rolling mill, but mainly to attain the required finished product parameters such as profile, contour and flatness, to monitor the mass flow, to control the mill so as to achieve the demanded final thickness, and to ensure minimum tolerances of these parameters. The mechanical properties of the finished strip are significantly affected by the finish rolling and coiling temperatures as well as the cooling strategy. Suitable physical process models have been developed and adapted them to ever-rising product requirements.
Hot-rolled strip produced by means of CSP technology, especially strip in the lower thickness range down to 0.8 mm, has begun to influence the design of the downstream technological process stages such as pickling and galvanizing. A typical example of a combined plant configuration is the hot-strip pickling and galvanizing line put on stream at Wuppermann in the Netherlands in 2000, which pickles and galvanizes hot strip in the thickness range of 0.8 mm to 3.0 mm. It is also possible to integrate one or two cold rolling stands into processing lines of this type.
In-line Strip Production process
In-line Strip Production (ISP) process produces hot-rolled coil down to finished gauges of 1 mm. One of the most striking characteristics of the ISP process is the overall compactness of the plant. With a line length of only 180 m from liquid steel to hot rolled coil, it is generally recognized as the shortest strip line in the world. This characteristic is the result of three significant ISP process features namely (i) continuous casting with liquid core reduction during slab solidification, (ii) direct linkage between steel casting and initial slab rolling, and (iii) the use of a compact induction heater combined with two coil box furnaces, rather than long tunnel furnaces at the entry side of the hot rolling mill.
The liquid steel is cast in a multi-bending mould with servo-hydraulic oscillation and an exit thickness of 70 mm. The slab undergoes soft reduction as it travels down the 5.2 m radius caster, to emerge at a speed of 5.5 m/min and at a maximum thickness of 55 mm. The tundish nozzle is designed to ensure homogeneous shell growth and the casting of long sequences.
Immediately on leaving the caster the slab enters a 3 stand roughing mill for reduction to a 10 mm to 18 mm thick transfer bar, which is then cut- to-length by a transverse pendulum shear. Liquid core reduction allows the production of a homogenous steel slab of high cleanliness, virtually free of segregation and with good grain refinement to give better mechanical characteristics to the finished steel. Also, the combination of liquid core reduction with the direct entry of the slab into the roughing mill brings energy saving advantages over conventional interrupted rolling sequences.
After passing through an induction heating furnace to raise the steel temperature by 150 deg C to 250 deg C, the transfer bar reaches the ‘Cremona furnace’. This unit comprises two coilers housed in insulated chambers, or boxes, and while one coiler is accepting and coiling a transfer bar arriving from the induction furnace, the other is decoiling the previous transfer bar to feed the hot-rolling mill.
Although the gas-fired Cremona furnace decouples the casting and roughing stage from the finishing mill, it is a dynamic buffer in that it holds the steel in-line and homogenizes the temperature profile ready for hot rolling, and hence in achieving of high productivity and efficiency. The resulting hot-strip edges extend work roll campaigns on the finishing mill upto 150 km.
The steel is descaled at high pressure before entering a 5-stand hot mill equipped with work roll shifting and bending plus automatic gauge control on all 4-high stands. A mill entry gauge of 20 mm or less enables Arvedi to produce hot-rolled strip down to 1 mm in a single pass with high profile and gauge precision, a crown level of 1 % to 3 %, low surface roughness and good cold deformability.
At this lower gauge, length deviations across the strip are below or equal to one ‘I’ unit. Finally, the rolled strip passes along a laminar water flow cooling table before down-coiling. A high level of process control and automation is an integral feature of the whole plant. Fume exhaust emissions are 1.0 mg / N cum, and total water recirculation on site avoids liquid discharges.
The dimensional tolerances of the hot-rolled strip are comparable with those of cold rolled product, with 1 mm to 1.2 mm gauge coil showing a flat transversal value when measured 25 mm in from the strip edge. This process is shown schematically in Fig 6.
Fig 6 Schematic of In-line Strip Production process
Thin slab casting and rolling technology from Danieli
The first generation process or thin slab casting and rolling process from Danieli consisted of flexible thin slab casting (fTSC) unit connected to thin slab rolling unit (fTSR) through a tunnel furnace. The fTSC unit was able to cast slab of thickness 60 mm. The caster was of vertical curved design, having funnel mould with soft reduction and air mist cooling. Rolling mill consisted of a finishing mill with 6 to 7 rolling stands in cluster configuration. This process is shown schematically in Fig 7.
Fig 7 Schematics of fTSC and fTSR process
Through continuous process development Danieli developed two different layout concepts and under these concepts the thin slab casting and rolling process was named as ‘Quality Strip Production (QSP), and ‘Quality Strip Production Endless (QSP-E) (Fig 8).
In the QSP, the plant with one or two casting strands is connected to the rolling mill through long tunnel furnace(s), which have the function of reheating and equalizing slab temperature as well as of guaranteeing sufficient buffer time in case of either scheduled stoppages of the mill (e.g., work roll change) or unscheduled interruptions of material flow. In recent years, thanks to a progressive increase in the mass flow due to increased casting speeds inherited from the last generation of thin slab casters operating at ultra-high speeds, and in combination with the reliable introduction of induction heating technology in place of tunnel furnaces, it was possible to develop the QSP-E configuration. This configuration resulted in extremely compact plants which are specifically dedicated to the production of ultra-thin gauges. This is made possible thanks to the application of the endless rolling process (i.e., the direct and uninterrupted connection between casting and rolling), to overcome the well-known problems of strip threading when producing thin gauges in coil-to-coil mode.
Fig 8 QSP and QSP-E processes
QSP plants can be operated with 2 casting strands which have made it possible to widen drastically the mix of thin slab steel grades and to ramp up plant productivity well in excess of 3 Mpta. These plants were initially conceived for the application of coil-to-coil rolling and then for the semi-endless process to roll thinner gauges below 1 mm. The QSP-E concept made it possible to optimize the production of ultra-thin gauges in endless mode, but showing little flexibility in the production of more sophisticated grades due to the rigid link between casting and rolling.
In the case of endless rolling, the caster is in fact forced to always run at very high casting speeds and this is not possible for all the steel grades for metallurgical reasons. Moreover, the endless process has proven to be economically competitive only for the production of coils having strip thickness below 1.5 mm, but as soon as the strip thickness is increased, the power required by the induction heaters to continue operating becomes excessively high, making it economically necessary to return to coil-to-coil mode.
In a continuous effort to improve existing processes and technologies and overcome their current limitations, Danieli has developed a new concept in TSCR plants. This concept is called Danieli Universal Endless (DUE). DUE concept is able to unify in a single production line all the winning features that upto now have been developed using different approaches, while eliminating the limiting factors of each one of them.
The DUE layout features (i) high productivity because of an unprecedented combination of slab thickness and speed, (ii) high production flexibility, able to operate in coil-to-coil, semi-endless and endless rolling mode, (iii) high operational flexibility, due to the presence of the tunnel furnace and relevant buffer time, (iv) covers the full spectrum of steel grades produced for flat products, including the most sophisticated ones rolled via thermo-mechanical rolling (API pipeline grades) or temperature-controlled rolling (multi-phase products) as well as the grades which require moderate casting speeds like peritectic, electrical steels and high carbon grades, being crack-sensitive, and (v) covers the full spectrum of geometrical strip dimensions, ranging from 0.8 mm ultra-thin gauges, produced in endless mode, up to 25 mm thick strips. This, of course, in combination with an unbeatable transformation cost, lower than any other process presently available.
The CONROLL process produces 70 mm to 80 mm thick slab through a straight mould with parallel sides but does not perform strand thickness reductions. High casting speeds in the range of 2 m/min to 4 m/min and width of 800 mm to 1600 mm is possible. The caster is connected to rolling mill via a roller hearth reheating furnace which equalizes the slab temperature to 1120 deg C. The rolling mill can include four, five, or six finishing stands depending on product mix and required finishing gauge. The rolling mill consists of a hydraulic shear mechanism, high pressure water descaler, laminar strip cooling system, and a downcoiler. Final strip thickness is in the range of 1.8 mm to 20 mm.
The CONROLL technology was installed in April 1995 at Armco’s Mansfield. Armco adopted this technology because it was designed specifically for stainless steel. Steel grades produced include carbon steel, stainless steel grades of 400 series 409 and 430, a small percentage of high strength low alloy (HSLA) grades, high alloyed steel grades and silicon steels.
Tippins Incorporated of USA teamed up with caster builder Samsung Heavy Industries of South Korea to develop TSP technology. This technology is suitable for low carbon to high carbon grades of steels, stainless steels, HSLA steels, silicon steel, API steel grades, and drawing quality steel. One of the strength of the mill is its versatility. It can produce coil or discrete plates enabling a wide range of width and gauges to be produced. The casting of intermediate thickness slabs has some advantages. The slab is thin enough to eliminate the need for a separate roughing mill and thick enough to maintain good quality. The intermediate thickness allows for greater slab width, reduced reheating time and hence reduced scale formation compared to 50 mm thin slabs.
After casting, slabs are sent to directly to reheating furnace to equalize the slab to the correct rolling temperature. The typical holding time to heat a slab to 1250 deg C is 12 min to 13 min. Once the strip has been reduced to 20 mm to 25 mm thickness, it is coiled to in the coiling furnace to retain strip temperature. The strip is then reversed back and forth through the rolling mill. A total of three flat roughing passes and six coiling finishing passes are generally required to finish gauges to 1.5 mm thickness.