Abstract:
The control of the recrystallisation during hot rolling of steel, is of paramount importance to manufacture products with ultrafine microstructure able to possess both high strength and high toughness. However, during conventional hot rolling, the recrystallisation process promptly takes place when the material is deformed and therefore, very fine microstructures cannot be achieved in the final product. The combination of thermomechanical controlled processing (TMCP) and the addition of niobium to steel, has proven to be a functional way to prevent the recrystallisation and subsequently refine the microstructure. A relatively new type of steel known as HTP (high temperature processing), was developed to manufacture the grade API 5L X80 used in transmission pipelines, this grade contains low carbon and high niobium level (around 0.10%) and can allow for controlled rolling at high rolling temperatures.
The effect of Niobium on recrystallisation of steel in TMCP has been widely studied, however, the understanding of the effect of the strain induced precipitation on the retardation of recrystallisation over the final finishing passes has not been investigated systematically, particularly in microalloy steels with high contents of Nb such as the HTP alloy. This research investigates an HTP concept steel composition, utilising plane strain compression testing (PSCT) to replicate commercial finishing passes and subsequent accelerated cooling. The aim of this work is to study diverse variables (strain, strain rate, deformation temperature, number of passes, holding time between passes and cooling rates) in a TMCP to simulate rolling routes able produce steels with ultrafine grain size.
Simulation and modelling of the interaction between the recrystallisation and precipitation at different strains and temperatures was carried out, the results showed that recrystallisation can be supressed even at substantially higher temperatures than those in the conventional rolling methods, the strain induced precipitation of Nb(C,N) played in fundamental roll in retarding the recrystallisation and consequently, unrecrystallised microstructures with highly elongated grains in the direction of the deformation where produced. An optimal austenite conditioning during rough and finishing rolling was achieved, it was shown that it is possible to perform series of deformation passes plus holding times without triggering the recrystallisation process.
Another set of experiments to produce micrometric ultrafine ferrite by TMCP was designed, when the HTP steel alloy was deformed at temperatures below Ae3 but above Ar3, abundant ultrafine ferrite was dynamically transformed during the deformation, analysis in SEM and SEM-EBSD were performed and it was observed that the size of the ultrafine ferrite ranged from about 0.5 to 2 microns.
