Principles of Conventional Controlled Rolling

Traditional hot rolling of steel takes place into the austenite region. As long as the temperature stays above a critical value, called “the temperature of no recrystallization (TNR)”, the austenite grains remain more or less equiaxed, due to subsequent wave of recrystallization on the run-out table in between the rolling passes. After hot rolling the austenite grains transform to ferrite and some pearlite during cooling of the hot rolled plate. Since most ferrite grains nucleate on the former austenite grain boundaries, a finer austenite grain size results in the finer pearlite. In the conventional hot rolling, the reheating temperature before entering the roughing mill is usually around 1200-1250 deg.C. In this carried out above TNR to avoid high rolling forces as a result of accumulation of strain hardening due to absence of recrystallization.

In Controlled rolling the reheating temperature is limited in order to avoid excessive austenite grain growth (eg. 1100 deg. C). the final rolling passes are carried out below TNR. This results in pancaked austenite with an increased grain boundary fraction/ volume and hence in more ferrite nuclei and smaller ferrite grains. The consequence is of course an accumulation of strain hardening in the last rolling passes, leading to high rolling forces. A common practice is to apply more reduction in the first passes (roughing mill) and to reduce the strain in the final passes.

Accelerated cooling starts above A3 and is maintained till about 500 deg. C. It activates additional ferrite nuclei inside the austenite grains and hence contributes to additional ferrite grain refinement. A faster cooling can however also lead to the formation of some bainite. This contributes to the strength of the material but it impairs the toughness. Accelerated cooling should be limited to 10-15 deg.C/s.

 Table 1 Main rolling parameters for conventional rolling and controlled rolling of a structural steel

 [Hot rolling of a slab from 220mm thick into a plate of 20mm (0.12wt%C, 1.4wt%Mn,0.025wt% Nb)]

Typical reduction per pass:

Roughing: 5-13%;  Finishing: - As per sequence

Conventional 25-35% Controlled Rolling 10-18%

The main differences between conventional and controlled rolling are the lower reheating temperature of the latter and the larger reduction during roughing. The time between roughing and finishing mill is much higher in controlled rolling in controlled rolling order to cool down the thick plate. This is considered as one of the drawbacks of controlled rolling because it decreases the productivity. In spite of the lower plate thickness, more rolling passes are needed in the finishing mill during controlled rolling, because the reduction per pass is limited due to the high rolling forces required at the temperature. After controlled rolling, an accelerated cooling of 8 deg. C/s is applied. The combined effect of controlled rolling and accelerated cooling is an increase of 50 MPa of the yield strength and decrease by 70 deg. C of the fracture appearance temperature.

keywords: Conventional Rolling, Roughing Mill; Finishing Mill; controlled Rolling; Fracture Appearance Transition Temperature(FATT); flow stress; yield strength; austenite; ferrite; grain boundary; Recrystallization Temperature