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Keywords:

  • microstructure;
  • recrystallization;
  • phase transformation;
  • mechanical properties

Abstract

Optimized microstructure and mechanical properties are the key requirements for line pipe steels. In the hot rolling process, it can be achieved through optimization in rolling schedule and cooling process. The present study uses hot strip mill modeling (HSMM) interface to model hot rolling process. Two API line pipe grades equivalent to “API X-70 & API X-80” have been modeled for its final microstructure and mechanical properties based on thermal, mechanical, and microstructural changes occurring during hot rolling. The model has been validated with plant data and test results. Austenite recrystallization model has been validated against calculated flow stresses from actual roll forces applied. Phase transformation model has been validated with the final microstructure and mechanical properties. The API X-70 grade shows strain accumulation (i.e., no recrystallization) in the finish rolling stage only, while the X-80 grade carries the strain accumulation from the roughing passes also, resulting in very fine grain ferrite. It is due to high retained strain before the phase transformation on run out table. The predicted properties by the model are in close proximity for multiple coils and different locations in the coil for both the grades.