Thermo-mechanical Processing of API-X60 grade pipe line steel

Abstract

The development of high strength pipeline steel grades is major concern for a large number of steel manufacturers. The mechanical properties of API high strength steel can be remarkably improved through proper alloy design and appropriate thermo-mechanical processing that can refine and homogenize the final microstructure. The API X 60 grades conventionally is of ferrite-pearlite microstructure. The high strength API X80 grades is of acicular ferrite/Bainite microstructure. The present objective is to obtain the acicular ferrite/Bainite micro structural constituents in the given API-X60 grade pipeline steel by thermo-mechanical processing. The thermo mechanical process involves heating to austenitization at 12000C for 20 min followed by deformation at temperatures of 9000C, 10000C & 11000C for different strain levels at 0.4,0.8 &1.2 with constant strain rate of 2 sec-1 in a deformation dilatometer. After hot deformation, the specimens were quenched in water to retain the state of hot deformed austenite. The metallographic and EBSD results reveal the formations of Acicular Ferrite in case of sample deformed at 9000C which is reported to have good combinations of strength & toughness and there is formation of bainite and martensite at 10000Cand 11000C deformed samples respectively. The critical point of stress and strain regarding to occurrences of DRX (Dynamic Recrystallisation) at each temperature was calculated from the flow curves based on the order of stress-strain curve and also the fraction of recrystallised volume in the sample which basically deformed at high temperature is calculated.