Phase transformation, microstructure, and mechanical properties of X100 pipeline steels based on TMCP and HTP concepts.

This work described the manufacture of hot rolled X100 pipeline steels at the laboratory based on thermomechanically controlled processing (TMCP) and high-temperature processing (HTP) concepts. Continuous cooling phase transformation was assessed by dilatometric tests for both steels. Microstructures were characterized using optical and electron microscopes to elaborate the evolution of mechanical properties with process parameters. The main microstructures for both steels were changed from granular bainite to lath bainite/martensite or acicular ferrite with increasing cooling rate. However, the TMCP steel always had lower transformation critical temperatures than the HTP steel. After hot rolling, the TMCP steel with predominantly acicular ferrite showed more excellent mechanical properties than the HTP steel with mainly granular bainite. In this sense, the combined addition of Mo and low Nb is superior to high Nb content for the alloy design of X100 pipeline steel. The optimum performance of TMCP steel can be obtained using the following process parameters: 820 °C finish rolling temperature, 460 °C final accelerated cooling temperature, and 25 °C/s cooling rate. The statistical analysis of precipitates showed that their type, morphology, and size distribution were similar for both steels, implying that the effect of high Nb content seems not to be exploited completely in as-rolled HTP steel. [ABSTRACT FROM AUTHOR]