Study on microstructure and mechanical properties of API X70 steel at various hot rolling conditions

In the present work, API X70 steel, a type of high strength low alloy (HSLA) steel, hot-rolled at different conditions and the effect of various hot rolling parameters on microstructure and mechanical properties investigated through thermomechanical control processing (TMCP). Optical and scanning electron microscopy were used for microstructural investigations and the tensile and hardness test utilized for mechanical characterization. The steel preheated at 1250 for 30 min and hot rolled with the speed of 10 rpm in the temperature range of 1100 to 730 through 3 (sample A), 4 (sample B), and 5 (sample C) passes and cooled in air. A total strain of 0.62 was imposed on all samples. In the 5 pass condition, a sample rolled with a finishing rolling temperature of 900 (sample D), another with a speed of 20 rpm, and the rest (sample E) rolled and quenched in water (sample F). Results showed that, except for sample E, all samples' microstructure consists of ferrite and pearlite but with different morphology. Sample E showed the least grain size. Due to the formation of the quasi-stable phases, sample F exhibited the highest strength/hardness and the lowest ductility. The best combination of strength and ductility was achieved in sample E.