Evolution of microstructure and precipitation state during thermomechanical processing of a X80 microalloyed steel

Abstract

a b s t r a c tA series of anisothermal hot torsion tests were carried out to simulate hot rolling on a high-strength low-carbon CMnNbMoTi microalloyed steel corresponding to an industrial X80 grade for pipeline construction. Mean Flow Stress was graphically represented against the inverse of temperature to characterize the evolution of austenite microstructure during rolling, which was also studied by optical microscopy and SEM on samples quenched from several temperatures. On the other hand, particles precipitated at different temperatures during rolling were analyzed by means of TEM using the carbon extraction replica technique and their size distribution and mean size were determined, as well as their morphology, nature and chemical composition. The effect of rolling temperature and austenite strengthening obtained at the end of thermomechanical processing on final microstructure and precipitation state was studied. Austenite strengthening was characterized by means of the parameter known as accumulated stress ( ). It was found that ferrite grains are finer and more equiaxed when the austenite is more severely deformed during finishing (higher values of ) but lower values of generate a higher density of acicular structures after cooling, which should improve the balance of mechanical properties. The increase in strength associated to acicular ferrite compared to polygonal ferrite is revealed by the higher values of Vickers microhardness measured on samples corresponding to low . On the other hand, (Ti, Nb)-rich carbonitrides can be found from reheating and their size keeps a constant value near 20-30 nm during thermomechanical processing. A second population of much finer (Nb, Mo)-rich carbonitrides whose size is close to 5 nm forms from lower temperatures, near 1000 • C. The accomplishment of two different levels of at the end of hot rolling schedule does not seem to introduce major differences in precipitation state before final cooling.

Strengthening Mechanisms in Thermomechanically Processed NbTi-Microalloyed Steel

“…The particle size, morphology, and chemistry observed in this work correspond to those reported for similar steel compositions and processing parameters. [16][17][18][19][20][21][22] The precipitate parameters and compositions varied with TMP schedule (Table I; Figure 5). For particles in the >20 nm size range, the average diameter was the smallest, and the number density and volume fraction were the highest for the 1248 K (975°C) deformation temperature TMP schedule, compared to other two schedules.…”

The effect of thermomechanical controlled processing on recrystallisation and subsequent deformation-induced ferrite transformation textures in microalloyed steels

“…This is a result of optimised thermomechanical controlled processing (TMCP) to provide the desired microstructure and crystallographic texture, leading to steels with excellent formability, strength and toughness [3,4]. TMCP, consisting of controlled hot deformation followed by controlled cooling, are used to maximise the benefits of the microalloy additions to the steels [5,6].…”

Thermomechanical processing route to achieve ultrafine grains in low carbon microalloyed steels

“…The improvement in mechanical properties is due to ferrite grain refinement, which maximises the surface area-to-volume ratio (S v ) of austenite grain boundaries and the deformation band density [3,4]. However, high energy consumption and high rolling loads are a limiting factor for TMCP.…”