Hot Rolling

The continuous finishing mill — typically 5, 6, or 7 four-high rolling stands arranged in tandem — performs the final thickness reduction from the transfer bar (25–60 mm) to the finished strip gauge (1.5–25 mm). Entry temperature at finishing stand F1 is 980–1,060 °C; exit temperature at the last stand (F5–F7) is controlled to the finishing temperature target of 820–920 °C for most grades.

Finishing temperature is the single most important rolling parameter for final product properties. For ordinary structural steels (S235, S275), finishing above 900 °C ensures full recrystallisation and a relatively coarse, equiaxed grain — high ductility but modest strength. For high-strength low-alloy (HSLA) steels and thermomechanically processed (TMCP) grades (S355, S420, S460 and above), finishing temperature is lowered to 780–860 °C — below the no-recrystallisation temperature (Tnr) set by niobium microalloy additions — so that the final rolling passes deform austenite without allowing it to recrystallise. This "pancaked" austenite transforms on run-out cooling to extremely fine ferrite, giving the combination of high strength (>450 MPa yield) and excellent toughness (Charpy impact at -60 °C) that defines modern HSLA structural and linepipe steels.

Automatic gauge control (AGC) systems maintain strip thickness to ±0.02–0.05 mm across the strip length using hydraulic roll force actuators that respond to thickness deviations measured by X-ray gauges between stands. Strip flatness is controlled by work roll bending (applying hydraulic force to the roll bearings to crown or flatten the roll gap) and by controlled thermal crown (differential cooling of the roll surface).

After the finishing mill, the strip travels across the run-out table (ROT) — a 50–150 m long conveyor of driven rolls with banks of laminar water cooling headers above and below the strip. The laminar cooling system applies controlled water flows to cool the strip from the finishing temperature (820–920 °C) to the coiling temperature (450–700 °C) at controlled rates of 10–100 °C/s depending on grade requirements.

Coiling temperature is the second most important rolling parameter after finishing temperature. Higher coiling temperature (650–720 °C) allows extended recovery and precipitation during slow cooling in the coil, producing lower-strength, higher-ductility strip (preferred for deep-drawing applications). Lower coiling temperature (450–550 °C) suppresses precipitation kinetics and increases dislocation density retained from rolling, producing higher-strength strip with finer precipitate distribution (preferred for HSLA structural grades and dual-phase steels).

Water cooling patterns on the ROT are programmed by a process control model for each grade: the cooling rate, the temperature pattern (continuous cooling vs. interrupted cooling with temperature holds), and the final coiling temperature are all specified. Modern ROTs with controlled cooling capability allow the same composition to produce multiple strength grades simply by adjusting the cooling profile — a powerful tool for product mix flexibility. The coiled strip (typically 15–35 t per coil) is transferred to the cooling bed for ambient cooling before dispatch or further processing in the cold mill.

Each finishing mill stand is a four-high configuration: two large-diameter backup rolls (1,200–1,800 mm diameter) support two smaller-diameter work rolls (550–850 mm diameter) that directly contact the strip. Roll force during finishing passes is typically 20–40 MN per stand — an enormous load requiring massive mill housings and sophisticated hydraulic position control.

Work rolls wear during rolling — the roll surface develops a characteristic wear profile over a rolling campaign (typically 100–500 t of steel per roll change). The worn roll surface is not flat: it develops a parabolic shape (the "crown") that varies across the roll length. Crown is the thickness difference between the strip centre and its edges — a roll crown of 40 µm in the work roll translates to approximately 10–15 µm strip crown (centre thicker than edges). Target strip crown is 10–40 µm for most products, depending on downstream application. Excessive crown causes flatness problems after cold rolling (wavy edges or centre buckle, because edges and centre have different stored strain from uneven reduction).

Three active shape control tools are used in the finishing mill:

Work roll bending: Hydraulic cylinders apply bending force to the work roll chock (bearing housing). Positive bending (bowing the roll to a convex crown) reduces strip crown; negative bending (concave crown) increases it. Roll bending adjustments are made automatically every scan cycle (100–500 ms) based on flatness meter feedback.

CVC (Continuously Variable Crown) rolls: Work rolls with a specially ground S-shaped profile (a sinusoidal axial variation in diameter). Axially shifting the pair of CVC rolls relative to each other varies the effective roll gap crown — by shifting 100–200 mm, the mill can change strip crown by 20–80 µm. CVC is the preferred mechanism for large crown corrections that are beyond the range of roll bending alone.

Strip flatness measurement: A shapemeter (a segmented measurement roll at the finishing mill exit) measures the tension distribution across the strip width by sensing the deflection of each strip segment independently. Non-uniform tension indicates flatness defects — wavy edges or centre buckle. The shapemeter signal drives closed-loop control of roll bending and CVC shift to maintain flatness within ±5 I-units (1 I-unit = 1×10⁻⁵ differential strain).