Meaning
Thermo-mechanical steel processing requires precise temperature limits during the final reduction pass through the rolling mill to control the grain structure of the plate. This threshold, known as finish rolling temperature, determines whether the deformation of austenite occurs above or below the recrystallization limit. It is monitored by optical pyrometers positioned immediately after the last rolling stand.
The selected value varies depending on the alloy composition and the required mechanical properties of the finished plate.
Process Control
Maintaining the correct thermal window ensures that the microstructural changes align with the target specifications. For high-strength low-alloy steels, the finish rolling temperature is kept within a range that suppresses recrystallization to accumulate strain in the austenite grains. This strain acts as a driving force for ferrite nucleation during subsequent cooling.
Deviations from this window produce non-uniform grain sizes.
Steel Microstructure
Deformed grains that do not recrystallize transform into fine, elongated ferrite crystals during cooling. Achieving a low finish rolling temperature promotes this grain refinement, which enhances both yield strength and low-temperature impact toughness. This structural refinement cannot be achieved through subsequent heat treatment alone.
It remains a permanent characteristic of the rolled metal.
Mill Operation
Rolling mills adjust the speed of the plate through the water sprays before the final passes to manage the heat loss. If the finish rolling temperature falls too low, the rolling force increases rapidly, which accelerates roll wear. This operational constraint limits the minimum temperature for processing.
