Meaning
Thermo-mechanical hot rolling of steel requires knowing the specific thermal limit below which deformed austenite grains do not undergo spontaneous recrystallization during the inter-pass time. This material property, designated as the non recrystallization temperature, marks the transition between dynamic recrystallization and strain accumulation. It is determined by the presence of microalloying elements like niobium and titanium in the steel composition.
The value of this temperature defines the start of the finish-rolling stage in the production cycle.
Metallurgical Process
Deformation of the metal below this thermal threshold causes the austenite grains to elongate and accumulate strain energy. In the region below the non recrystallization temperature, the strain is retained in the form of dislocations and deformation bands. These defect structures serve as dense nucleation sites for the subsequently formed ferrite phase.
This mechanism produces a highly refined grain structure.
Processing Window
Mill operators adjust the rolling passes to ensure that the final deformation occurs below this limit. If the metal is worked above the non recrystallization temperature, the grains recrystallize between passes, which prevents the accumulation of strain. This failure results in a coarser and less uniform grain structure.
The processing schedule must be carefully monitored to avoid this.
Mechanical Impact
Grain refinement resulting from deformation in the low-temperature regime enhances the yield strength of the steel plate. This processing route achieves high strength and excellent fracture toughness without the need for post-rolling heat treatment. This makes the finished plates highly cost-effective for construction.
