Abstract
AbstractA CALculation of PHase Diagrams (CALPHAD) approach was used to study the precipitation of nitrides and carbonitrides in pipeline steels, aligned with new developments of complex chemical compositions and thermomechanical processing of High Strength Low Alloyed (HSLA) Steels. This is in response to growing demand for improved mechanical and chemical properties, manufacturing flexibility and reduced production cost. The calculated results indicated that the precipitation temperatures of nitrides in Ti‐Nb microalloyed steels increased by titanium concentration, while the niobium concentration significantly increased the precipitation temperature of niobium carbonitrides. Carbonitride precipitates formed at much lower temperatures (∼100 K) in low carbon steels (<0.03 wt%) than the high carbon steels (>0.1 wt%), suggesting precipitates larger in size. This is in good agreement with independent experimental data from the literatures, where austenite grain growth was studied in similar steel compositions. Although the dissolution and growth of precipitates are controlled kinetically, these results proved that the thermodynamic calculation can efficiently predict compositions and sequence of precipitation in chemically complex systems, guiding more accurate designs of experiments to identify critical temperatures of grain coarsening during reheating, recrystallisation during hot rolling, and transformation during cooling. This can minimize the number of tests required to obtain optimum chemical compositions and heat treatment procedures.
Highlights
IntroductionYAMINI compared to carbon steels.[1,2,3] Nitrides,[4,5] carbides,[6,7] and carbonitrides[5,8] of microalloying elements, such as niobium, titanium, and vanadium, precipitate in austenite during thermomechanical processes, pin grain boundaries and improve the mechanical properties of the steels through grain refinement and precipitation hardening.[9]
Several industrial compositions of pipeline steels were selected to calculate the phase diagrams and study the composition and formation temperature of nitride and carbonitride precipitates, aiming to explore the possibility of calculated methods to minimize the number of experiments required to develop new chemical compositions and the optimum thermomechanical processing for microalloyed steels
The ThermoCalc software package, using the thermodynamic databases of TCFE7 was employed to study the precipitation of nitrides and carbonitrides as functions of C, N, Ti and Nb concentrations in pipeline steels
Summary
YAMINI compared to carbon steels.[1,2,3] Nitrides,[4,5] carbides,[6,7] and carbonitrides[5,8] of microalloying elements, such as niobium, titanium, and vanadium, precipitate in austenite during thermomechanical processes, pin grain boundaries and improve the mechanical properties of the steels through grain refinement and precipitation hardening.[9]. Depending on the chemical composition of the steel, above certain temperatures during heat treatment, rolling and/or welding, the austenite grains tend to coarsen because the precipitates are taken into solution, leaving the grain boundaries unimpeded.[10,11,12] Formation and coarsening of nitride and carbonitride precipitates, in microalloyed steels, have been extensively studied experimentally due to their influence on the mechanical properties of products,[1,4,8,13] the formation of cracks during shaping of steels[14,15,16] and corrosion resistance due to hydrogen induced cracking[2,13] and microstructural texture.[2,17,18] due to the growing demand for improved mechanical properties, environmental resistance, weldability and reduced production cost of High Strength Low Alloy (HSLA) steels, new complex chemical compositions and thermomechanical processing have been continuously developed to advance high-grade pipeline steels for practical applications.[3,19,20,21,22] This highlights the needs for better understanding of precipitation behavior including sequence of precipitation in various chemical compositions of microalloyed steels
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