Hot Rolling of Steel: Recrystallisation, Controlled Rolling, and Microstructure Development

Introduction to Hot Rolling

Hot rolling is the primary shaping and microstructure-control process for structural steels, linepipe grades, heavy plates, and structural sections. The rolling schedule — the sequence of reduction passes, inter-pass times, rolling temperatures, and post-rolling cooling — determines the final ferrite grain size, strength, toughness, and formability of the product. Modern thermomechanical controlled processing (TMCP) exploits precise control of rolling and cooling to achieve properties impossible with conventional reheating and rolling.

Metallurgical Events During Hot Rolling

At hot rolling temperatures (950–1,250°C), the steel is in the fully austenitic condition. Three metallurgical events occur during rolling:

1. Dynamic Recrystallisation (DRX)

DRX occurs during a rolling pass when strain exceeds a critical value (ε_c ≈ 0.5–0.8 depending on Z-parameter). New, strain-free grains nucleate within the deformed grains and grow during the pass. DRX is favoured at high temperatures and slow strain rates. It is the dominant softening mechanism in high-temperature rolling passes on continuous tandem mills where passes follow rapidly.

2. Static Recrystallisation (SRX)

SRX occurs between rolling passes, driven by stored deformation energy. New grains nucleate at deformed grain boundaries and grow. For low-alloy steel without microalloying, SRX completes in seconds at 1,100°C. The recrystallised grain size depends on prior deformation and temperature.

3. Grain Growth

After recrystallisation is complete, grains grow to minimise boundary energy. Grain growth rate is strongly temperature-dependent. Microalloying precipitates (TiN, NbC/N) pin grain boundaries (Zener pinning) and limit grain coarsening — the primary purpose of titanium addition (0.01–0.02% Ti) in modern HSLA steels.

The Non-Recrystallisation Temperature (T_nr)

Below a critical temperature T_nr, niobium in solid solution or as fine NbC/N precipitates retards SRX between passes — austenite accumulates strain without recrystallising. This is the key phenomenon exploited in TMCP.

T_nr (°C) = 887 + 464×%C + (6645×%Nb − 644×√%Nb) + 732×%V − 230×√%V + 890×%Ti + 363×%Al − 357×%Si

For typical Nb-HSLA linepipe steel (0.07C, 0.035Nb): T_nr ≈ 950°C. Rolling below this temperature deforms austenite without recovery, building up a “pancaked” elongated grain structure with high dislocation density — the TMCP hot zone.

TMCP Rolling Schedule Design

A TMCP schedule for heavy plate production (e.g. API 5L X70 linepipe steel) typically follows these stages:

1. Slab reheating: 1,150–1,200°C to dissolve Nb into austenite solution (TiN remains undissolved for grain boundary pinning)
2. Roughing passes (above T_nr): Multiple passes at 1,050–1,150°C with SRX between passes — refines austenite grain size from ~200µm (as-reheated) to ~50–80µm
3. Intermediate cooling: Let plate cool to just above T_nr (950–980°C for Nb steels)
4. Finish rolling (below T_nr): 3–6 passes between 850–950°C achieving total reduction 50–70%. Austenite grains become elongated “pancakes” with aspect ratio 5–10:1 and dislocation density 10¹⁵ m⁻²
5. Accelerated cooling (ACC): Water sprays from 800–850°C down to 450–600°C at 15–30°C/s. Ultrafine nucleation of ferrite from pancaked austenite produces ASTM grain size 10–13
6. Controlled cooling stop temperature: Determines bainite fraction for additional strength. Higher strength (X80, X100) grades cool further into the bainite field

Texture and Anisotropy in Hot-Rolled Steel

Hot rolling in the non-recrystallisation region not only refines grain size but also develops crystallographic texture. The deformation texture in hot-rolled steel plate produces anisotropy:

• Higher yield strength in the rolling direction (RD) than transverse direction (TD) — typically 15–30 MPa difference in plates rolled below T_nr
• Lower Charpy impact values in the short transverse direction (through-thickness) compared to RD or TD — a critical consideration for offshore structural steels where through-thickness loading occurs
• Improved resistance to through-thickness lamellar tearing in Z-quality plates (achieved by low sulphur + calcium treatment to control inclusion morphology)

Frequently Asked Questions

Conclusion

Hot rolling of steel is not merely a shaping operation — it is the primary microstructure-control process for modern structural and linepipe steels. The combination of controlled deformation below T_nr (exploiting Nb microalloying), accelerated cooling, and precipitation strengthening from V and Nb allows TMCP to produce X65–X80 linepipe steels with a combination of strength, toughness, and weldability impossible by any other processing route. See also: HSLA Steels and Microalloying and Grain Refinement and Hall-Petch Strengthening.

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