Modern Strip Production Technologies

Introduction

The strip casting process that can produce steel strip directly from molten steel thus bypassing the hot rolling process has been a dream of steel engineers since Henry Bessemer published his theory in 1856. Since the late 1980’s the development or near net shape casting regained momentum, and this lead to the revival of twin-drum (or twin-roll) casting machines of the Bessemer type.

Compact Strip Production (CSP) is a novel technology developed in 1980s for casting-hot-rolling of thin slabs. It is new technological innovation following converter steelmaking and continuous casting technology in steel industry. This technology provides a more compact line and a more simple procedure as compared with the traditional hot rolling technology.

The Main Technical Features of CSP

The CSP concept is based on the designs of Schlömann Siemag Ag (SMS) in Germany, where it was tested on a pilot plant basis starting in 1985. Marketed by SMS/Concast, the first commercial unit was ordered by NUCOR late in 1986, and construction was started at Crawfordsville, Indiana, in the fall of 1987. In July 1989, the first compact strip production (CSP) plant started up and a year later, in June 1990, profitable operations were first attained. The steelmaking section of Nucor’s CSP plant consists of two ABB-supplied, Fuchs constructed electric arc AC furnaces with water cooled side walls and roofs, oxy fuel burners, and eccentric bottom tapping.

After steelmaking, the liquid is teemed into the tundish of the caster, after which it is solidified to the desired thickness of 40 to 70 mm. The slab is then sheared to the proper length and transported to the tunnel or equilibrating furnace normally set at 1150°C. At this point, the slab exhibits an austenite grain size of 500-1000 μm.

After the 20 minute residency time in the furnace, the slab exits the furnace, is crop sheared, and then enters the finishing mill at approximately 1000°C. After the slab passes through the finishing mill of 5, 6 or even 7 stands, it enters the run-out table (ROT) where it undergoes cooling to the coiling temperature, after which it is coiled to room temperature. This process is shown schematically in Figure 1.

Figure 1: Schematic diagram of an integrated production line of the hot-rolled plates in the CSP (Compact Strip Production) process.

As mentioned above, the most CSP plants have 6 stands, with small portion of them which use 5-stand or 7-stand layouts. CSP plants come with profile and flatness control systems adopting the well known CVC technology for adjustment.

The systems enable defined profile heights to be set for the finished strip irrespective of the distribution of pass reductions in the mill. Elastic deformation and thermal crown and roll wear are assessed by the profile and flatness control system and compensated when presetting the roll gaps.

Compensation for roll gap presetting for the undesired profile and flatness resulting from:

• Changes in the elastic deformation of the set of rolls
• Changes in the thermal crown
• Changes in the roll crown caused by wear.

For Nucor’s installations, a newer technology called PFC-CFC was available, based on the combination of the contour and flatness control with the so-called contour and flatness control systems. The contour and flatness control system is designed for the purpose of eliminating any profile anomalies resulted roll wear and thermal crown. Such roll wear and thermal crown are likely to occur when rolling is done either with constant width or with changing widths over extended periods of time.

For high pressure de-scaling ahead of the rolling mill, there are 400 bar variable pressure pumps for recent installations, or 250 bar high pressure pumps for older ones. If requested, for example for rolling some sensitive grades, a roller side guide (edger) is available to be installed in front of stand F1 for improved guidance and edge quality, etc.

In most plants, production capacity is about 1 million tons per year, ranging from 0.9-1.5 million tons/year. Slab dimensions range from 40 to 70 mm in thickness and 900 to 1600 mm in width in the most of installations. Minimum finish size of strip is from 1.2 mm to 2 mm in most cases; newer mills usually have thinner finish size. Heat size usually is 120 to 180 tons, ranging from less than 100 tons to over 200 tons.

In-line Strip Production

Almost at the same time developing of CSP process, the In-line Strip Production (ISP) was under intensive development by then Mannesmann Demag AG and Arvedi Group. Now, the both technologies have since long achieved worldwide recognition.

Though there are slight differences in the layouts of the two strip production technologies, they both cast the steel into a thin gage, reheat it, and roll it into finish gage in a limited number of passes, thus greatly reducing the production stages and lower the energy consumption. Continuous casting and rolling steel strip was a revolution in steel industry at the end of the last century. Some thin gauges that conventionally have to be produced by cold rolling, can be obtained now from hot rolling with the new CSP or ISP process, which can roll 1.2 mm now and maybe 0.7-0.8 mm in the future.

One of the most striking characteristics of the ISP process is the overall compactness of a plant. A plant with a line length of only 180 m from liquid steel to hot-rolled output today is 50% above its original design capacity of 0.6Mt/yr. However, the extended commissioning at Cremona (Italy) combined with the declining financial fortunes of Mannesmann Demag hampered sales opportunities for ISP technology during a period of global mini-mill expansion in the early- to mid-1990s; nevertheless several other ISP installations are now operational. For example, Posco’s Kwangyang works in South Korea operates a twin-strand ISP, while Saldanha Steel in South Africa and Corus’s Ijmuiden works in the Netherlands are equipped with single strand units.

An Example of Plant Features

The production of steel at Acciaieria ISP di Cremona commences with the charging of a 110t, 110MVA EAF equipped with eccentric bottom tapping, using scrap stored in a covered scrap yard. There is an average of 25 heats per day, and liquid steel is refined in two 18MVA ladle furnaces before casting in a multi-bending mould with servo-hydraulic oscillation and an exit thickness of 70mm.

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 10–18 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 150–250°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.

The steel is de-scaled 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 20mm or less enables Arvedi to produce hot-rolled strip down to 1mm in a single pass with high profile and gauge precision, a crown level of 1–3%, low surface roughness and good cold deformability.

The dimensional tolerances of the hot-rolled strip are comparable with those of cold rolled product, with 1–1.2mm gauge coil showing a flat transversal value when measured 25mm in from the strip edge.

Tests on the finished product by other steelmakers around Europe have confirmed its suitability for galvanizing, shaping, welding and cold rolling, and reports from two German research institutes note the high degree of consistency in the mechanical characteristics, the low surface roughness, the crystalline structure, and the similarity of the steel formability curve to that of a cold-rolled product.

Figure 2: Crystal lattice structures and cryogenic tempering.

A great feature of the ISP is the short production circle compared with the traditional processes and the cast-rolling capability compared with other new processes. The overall ISP plant is only 180 meter long from liquid steel to finished coil, with a production cycle of only 15 minutes. Production capacity is in the range of 0.7-0.9 million tons for one casting line, and up to 2 million tons for two casting lines. Several key features of ISP are summarized in the Table 1.