Development of technology for production of heavy plates from continuously cast slabs of low-carbon microalloyed steels using thermomechanical treatment

A set of laboratory and industrial experiments was conducted to study the effect of thermomechanical treatment (TMT) on the structure of low-carbon microalloyed steels. Based on the revealed regularities of structural transformations under the influence of TMT, the features of the formation of the actual microstructure across the thickness of heavy plates were established. The processes of secondary recrystallization of ferrite on the rolled product surface, which have a negative effect on the low-temperature impact toughness, were studied. It was shown that austenitization at a temperature of 900 °C before deformation and the formation of fine austenite grain minimize the negative effect of secondary recrystallization of ferrite on the dispersion of the final microstructure. The effect of austenite grain and the post-deformation cooling rate on a set of mechanical properties was demonstrated. For the conditions of rolling mill 5000 of United Metallurgical Company, technologies for the production of steel heavy plates up to 130 mm thick of strength classes C355-C440 (S355-S460) were developed and mastered through the use of various TMT options, including those with regulated impact toughness indicators down to -60 °C. The results of multi-stage thermomechanical treatment, including double austenitization of the slab, are presented, which ensures the formation of a homogeneous dispersed microstructure, mechanical properties and impact toughness at –80 °C throughout the entire thickness of the heavy plates.