Effect of Hot Rolled Process on Microstructure and Properties of X70 Pipeline Steel

The microstructure and mechanical properties were studied on X70 pipeline steel strips on 1700 Hot Mill produced by Tangsteel using methods of optical microscope, SEM and TEM tensile tests and so on. And effect of impurities in the steel on impact toughness was researched. The results show that microstructure of X70 pipeline steel strips is mainly acicular ferrite of which the grains are about 6μm. Finer M/A island structure and deteriorated pearlite are dispersed in the lathing of acicular ferrite. The impurities in steel are mainly sphericity, subspheticity of silicates and sulfuret, and the size of most impurities is smaller than 5μm. The strength and toughness of X70 pipeline steel strips are high because of the fine acicular ferritic microstructure and low quantity of impurities.