Hot Rolling

Hot rolling is the first mechanical deformation step in transforming continuously cast steel into a useful semi-finished or finished product. Slabs (150–350 mm thick) are reduced to strip of 1.5–25 mm thickness, plates of 6–150 mm, or structural sections through a series of rolling passes at temperatures above the steel's austenite recrystallisation temperature (typically 900–1,250 °C). At these temperatures, steel flows plastically with forces 3–5 times lower than at room temperature, enabling the large thickness reductions — 90–97% total reduction for thin strip — that are the essence of the process.

The hot strip mill (HSM) — the most common configuration for flat-rolled products — is a linear sequence of equipment stretching 500–700 metres: a walking beam reheating furnace, a descaling station, a reversing roughing mill, a transfer table, a coilbox (optional), a continuous finishing mill of 5–7 stands, a run-out table with laminar cooling, and a downcoiler. Entry slab temperature is 1,150–1,280 °C; the strip exits the finishing mill at 820–920 °C and is coiled at 550–700 °C. The entire rolling sequence for a 250 mm slab being reduced to 3 mm strip — a thickness reduction of 99 to 1 — takes approximately 3–5 minutes.

Hot rolling creates not only the final thickness but also the mechanical properties of the strip through controlled thermomechanical processing. The finishing temperature, reduction schedule, and coiling temperature collectively determine the steel's grain size, precipitation state, and dislocation density — and therefore its yield strength, tensile strength, toughness, and formability.

Cast slabs must be reheated to 1,150–1,280 °C before rolling to dissolve microalloying precipitates (Nb, Ti, V carbides and nitrides formed during casting) and to achieve the uniform temperature required for consistent rolling force and product properties. Reheating is performed in walking beam furnaces — large gas-fired enclosures where slabs advance on water-cooled walking beams at a rate matched to the mill's rolling cycle, typically 200–500 t/hour throughput.

Furnace temperature profile is critical: a slab charged at 20–600 °C (cold or hot charge) must reach a uniform through-thickness temperature within ±25 °C to avoid uneven rolling and shape defects. The soaking zone at 1,200–1,280 °C provides the final equalization; slabs spend 1.5–4 hours in the furnace depending on thickness and charge temperature.

Scale formation on the slab surface is an unavoidable consequence of high-temperature reheating — the oxide layer (Fe₂O₃/Fe₃O₄/FeO) grows to 2–5 mm thickness and must be removed before rolling or it is pressed into the steel surface as scale pits. Hydraulic descalers — high-pressure water jets at 150–300 bar — break and wash off the scale before the roughing mill and between finishing stands. Primary descaling removes 70–80% of scale; secondary descaling between mill stands removes the thin oxide layer that regrows in transit from stand to stand.

Hot charging — delivering slabs directly from the caster at 600–900 °C without full cooling — saves 1.0–1.5 GJ/t of reheating energy and reduces scale loss from 1.5–2.0% to 0.8–1.2% of slab weight. Hot charging is now standard at plants where the caster and hot strip mill are collocated.

Iron oxide scale forms on the slab surface during reheating through a well-characterised three-layer structure:

Outer haematite (Fe₂O₃): The thin outermost layer, formed at the surface in direct contact with the furnace atmosphere. Relatively thin and brittle — the first layer to spall during descaling.

Middle magnetite (Fe₃O₄): The thickest middle layer, constituting 70–80% of total scale thickness. Harder and more adherent than haematite. The primary target of hydraulic descaling.

Inner wüstite (FeO): The innermost layer in direct contact with the steel. Grows by outward diffusion of iron ions through the scale. At the furnace exit temperature of 1,200–1,280 °C, wüstite is the dominant scale phase (>90% by volume). Wüstite has a melting point of approximately 1,380 °C — during reheating, liquid wüstite can penetrate austenite grain boundaries, causing "burnt steel" which is irreversibly damaged.

Scale growth follows a parabolic law — thickness ∝ √time at temperature. A slab spending 2 hours at 1,250 °C develops approximately 2–4 mm of scale. This represents a mass loss of 1.5–2.0% of slab weight for a cold-charged slab. Hot-charged slabs (entering the furnace at 600–900 °C and spending only 45–90 minutes to temperature) develop only 0.8–1.2% scale loss — a significant yield improvement.

Hydraulic descaling at 150–300 bar removes scale through two mechanisms: thermal shock (the cold water creates a sudden temperature drop that causes the brittle scale to crack and delaminate) and mechanical impact (the high-velocity water jet mechanically dislodges the cracked scale). Despite effective primary descaling, a thin layer of scale (secondary scale) regrows on the strip surface within 5–20 seconds at the roughing and finishing mill temperatures. Between finishing mill stands, each inter-stand period generates tertiary scale 50–200 µm thick. Tertiary scale is harder to remove than primary scale because the strip moves at 5–15 m/s between stands and there is insufficient time for a dedicated descaling pass. This tertiary scale is the primary source of "red scale" surface defects — reddish smears of pressed-in oxide particles on the strip surface. For automotive and appliance grades requiring a clean surface, the hot-rolled strip must pass through a pickle line (acid descaling in HCl or H₂SO₄) before cold rolling.

The roughing mill performs the first large-thickness reductions, reducing the 150–350 mm thick slab to a 25–60 mm thick "transfer bar" in 5–9 reversing passes. Roughing mills are typically a single reversing stand (the slab passes back and forth, with the roll gap closing on each pass) or a pair of roughing stands. Entry temperatures at the roughing mill are 1,100–1,200 °C; exit temperatures after 5–9 passes are 1,000–1,080 °C.

Edger rolls alongside the roughing mill (vertical rolls pressing on the slab edge) control slab width. The edger is critical for width accuracy: slab width spread occurs naturally during horizontal rolling (the material takes the path of least resistance and widens), and the edger compensates by controlled width reduction. Automatic width control (AWC) closed-loop systems using laser width gauges maintain final strip width to ±2–3 mm.

A coilbox — an optional unit between the roughing mill and finishing mill — coils the hot transfer bar into a coil to conserve heat, then uncoils it head-first (so the leading end, which has cooled most, enters the finishing mill first — ensuring more uniform temperature across the transfer bar length). Coilboxes are particularly valuable for thin transfer bars (producing thin strip <2 mm) where temperature drop over a long transfer table would cause the bar to cool below the critical finishing temperature before the tail end enters the finishing mill.