Process Overview
Hot rolling is the primary shaping process for steel semi-finished products. Slabs (for flat products), blooms (for sections and heavy bar), and billets (for rod and bar) emerging from the continuous caster are either fed directly to the rolling mill while still hot — in endless or direct rolling configurations — or cooled, stockpiled, and reheated in a furnace before rolling. In a conventional hot strip mill, a slab entering the roughing mill at 1,150–1,250 °C is reduced from 220–250 mm thick to 25–45 mm in the rougher (5–7 passes in a reversing or tandem arrangement), then passed through the 6–7-stand tandem finishing mill which reduces it to 1.2–25 mm at speeds up to 20 m/s, before being coiled at 550–750 °C on a downcoiler. The whole process from furnace discharge to coiler takes less than 5 minutes.
Reheating Furnace
Slabs are reheated in a walking-beam or pusher furnace to achieve a uniform temperature of 1,200–1,280 °C throughout the cross-section before rolling. Heating time is typically 2–4 hours depending on slab thickness and the initial temperature. The furnace atmosphere is controlled to minimise scale formation — excessive scale causes surface defects (pits) and represents a yield loss of 1–2% of slab weight. Descalers — high-pressure water jets at 150–350 bar — remove primary scale at the furnace exit and between rolling passes. Homogenising temperature is critical for microalloyed (HSLA) steels: undissolved niobium and vanadium carbides/nitrides must be brought into solution at this stage to be available for controlled precipitation during rolling and cooling.
Roughing Mill
The roughing mill reduces the slab from its as-cast thickness (typically 220–250 mm) to a transfer bar of 25–45 mm in 5–7 passes, usually in a single reversing stand (R1) or a fixed + reversing combination. Entry temperature at the rougher is 1,150–1,250 °C; exit temperature after roughing is typically 1,020–1,100 °C. Scale is removed between each pass by edger rolls (to control width) and high-pressure water. On wide hot strip mills, the roughing mill also sets the final width of the strip via edger rolls, which must compensate for spread during rolling. Transfer bar crop ends (head and tail) are sheared off before the finishing mill to remove the chilled, off-gauge ends that would cause threading problems.
Finishing Mill and Thermomechanical Processing
The tandem finishing mill — typically 6 or 7 four-high stands — reduces the transfer bar from 25–45 mm to final gauge at high speed with precise tension control between stands. Finishing temperature (the temperature at the exit of the last finishing stand) is a critical process parameter: for conventional plain carbon steel, 850–920 °C is targeted to ensure the steel is above the Ar3 transformation temperature (fully austenitic) during deformation. For thermomechanically controlled processed (TMCP) high-strength low-alloy (HSLA) steels — used in pipeline, shipplate, offshore, and structural applications — rolling is deliberately completed in the low-temperature austenite range (780–850 °C) where austenite is work-hardened and recrystallisation is suppressed by microalloying elements (Nb, V, Ti). This creates a fine, pancaked austenite grain that transforms on cooling to a fine ferrite-pearlite or bainitic microstructure with enhanced strength and toughness.
Runout Table Cooling and Coiling
Between the final finishing stand and the downcoiler, the hot strip passes over a runout table (typically 60–100 m long) where laminar water cooling headers apply controlled water flows to reduce the strip temperature to the coiling temperature target — typically 550–750 °C for most carbon grades. Coiling temperature controls the phase transformation and precipitation reactions that establish the final mechanical properties of the as-rolled product: higher coiling temperatures (680–720 °C) favour coarser ferrite and carbide dissolution for deep-drawing grades; lower coiling temperatures (480–560 °C) produce fine bainitic structures for high-strength structural and HSLA grades. Cooling rate uniformity across the strip width is critical to avoid shape defects (edge waves, centre buckle) caused by differential thermal contraction.
Typical Hot Strip Mill Parameters
| Parameter | Typical value |
|---|---|
| Slab dimensions | 220–250 mm × 900–2,050 mm × 6–12 m |
| Reheating temperature | 1,200–1,280 °C |
| Rougher exit thickness | 25–45 mm |
| Finishing gauge range | 1.2–25 mm |
| Finishing entry speed | 0.5–1.0 m/s |
| Finishing exit speed | 10–20 m/s |
| Finishing temperature | 840–920 °C |
| Coiling temperature | 480–750 °C (grade-dependent) |
| Coil weight | 15–35 t |
| Hourly production | 300–500 t/hr |
Operating parameters for a modern 2,050 mm wide hot strip mill.
Plate Mills and Section Mills
Not all hot rolling occurs on a continuous hot strip mill. Heavy plate mills roll slabs to discrete plates of 6–150 mm thickness and up to 5 m width for offshore structures, pressure vessels, shipbuilding, and heavy machinery — applications requiring impact toughness at low temperatures that are achieved through TMCP and accelerated cooling on the run-out table. Section mills roll blooms to I-beams, channels, angles, and rails. Rod and bar mills roll billets at high speed (up to 120 m/s for 5.5 mm wire rod) in continuous reducing passes through a sequence of oval and round passes, finishing in a no-twist block to achieve the dimensional tolerances required for downstream drawing and cold-forming operations.