EFFECT OF COOLING RATES AND INTERMEDIATE SLAB BLANK THICKNESS ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF THE X70 PIPELINE STEEL

In this study, the microstructures and mechanical properties of X70 pipeline steels produced with varying Mo contents, accelerated cooling rate and intermediate slab blank thickness are systematically investigated. Results showed that the microstructures and mechanical properties of the X70 pipeline steels were strongly affected by Mo addition. The pearlite and proeutectoid ferrite formation is obviously inhibited in containing-Mo steel and the acicular ferrite (AF) is obtained in a wide range of cooling rates. With the increasing the cooling rates, the AF constituent amount increases. The grains can be refined by increasing the thickness of intermediate slab for enhancing the cumulative reduction rates, and meanwhile increase the number density of precipitates. It was proved by simulation and industrial trials that the low-alloy X70 pipeline steels can be produced increasing cooling rates and the thickness of intermediate slab without strength and toughness degradation which also reduce alloy cost.